Sentence Correction

Interesting discussion. Rather a very common discussion point. Coincidentally, I covered this question in my live session just yesterday!!

force5 has given a very good assessment of like vs as. Here is a summary of the usage and a handy framework.

Usage
1: Like and As are used to state comparisons.
2: Like is used to compare NOUNS - Like entity A, entity B does xyz
3: As is used to compare actions/clauses - As entity A does, entity B does xyz

In certain cases the meaning of 2 = 3 but in certain other cases the meaning of 2 is different from the meaning of 3.

Case when meaning of 2 = 3
While watching her favorite show, the baby concentrates like an adult.
While watching her favorite show, the baby concentrates as an adult does.

Case when meaning of 2 is different from meaning of 3
The participant on American Idol performed like Celine Dion.
The participant on the dance show performed as Celine Dion performs.

Does GMAT test this different in meaning?
I have not seen evidence of that. That is - I have not seen an official sentence in which the two sentences above will be two choices and you would be asked to select the correct one. Both are correct but they communicate slightly different meaning.

Does the above mean that "As" cannot be followed by a noun?
No it does not mean that. The above implies that "as" cannot be followed by a noun when you want to state a comparison. As can very well be followed by a noun if you want to state the meaning "in the role of" or "function".

This brings us to another usage of As
Usage of As to show "function"
As role, entity A does xyz
Here A = role

As your teacher, I have the responsibility to impart knowledge to my students.
This sentence implies that "I am your teacher".

Note here - in this sentence, there is no comparison. The meaning of the sentence is that "I am your teacher..."

Framework
Step 1 - What is the intended meaning of the sentence? Does the sentence intend to communicate a comparison or does it intend to state that x plays the role of y.
Step 2 - If the comparison is to be stated then ensure that it is stated using one of the following ways:
Like + NOUN
As + CLAUSE

If the intended meaning is to show "role" or "function", then use the expression:
As + Noun

(Note that there are other uses of "As" as well - to indicate "during or while" or to indicate "reason". But these usages do not cause confusion with the usage of like.)

How does this Framework apply to this question?

Step 1 - Meaning of the Sentence:

Proponents of artificial intelligence say they will be able to make computers that can understand English and other human languages, recognize objects, and reason as an expert does-computers that will be used to diagnose equipment breakdowns, deciding whether to authorize a loan, or other purposes such as these.

Sentence talks about what the AI will make possible. Computers will be able to ....reason as an expert does....It is not logical to say that computers will play role of expert. The logical meaning here is the comparison - Computers and experts are being compared.

Step 2 - Correct expression
In the context of this sentence, both are correct:
Computers will be able to reason like an expert
Computers will be able to reason as an expert does

Thus, in this question even though there is a pretty clear split in the answer choices - like an expert or "as an expert does", this difference cannot be used to eliminate any answer choice.

Few other Official Questions:
GMATPrep
As with those of humans, the DNA of grape plants contains sites where certain unique sequences of nucleotides are repeated over and over.
A: As with those of humans, the DNA of grape plants contains sites where
B: As human DNA, the DNA of grape plants contain sites in which
C: As it is with human DNA, the DNA of grape plants, containing sites in which
D: Like human, the DNA of grape plants contain sites where
E: Like human DNA, the DNA of grape plants contains sites in which

In this sentence as well, the intended meaning is the "COMPARISON". Once you know this, you would know that choice B is incorrect.

Verbal Review Question 32
Like Rousseau, Tolstoi rebelled against...
A: Like Rousseau, Tolstoi rebelled
C: As Rousseau, Tolstoi rebelled

In this sentence as well, the intended meaning is the "COMPARISON". Once you know this, you would know right away to eliminate choice C.

But now lets assume the sentence were a little different:
Tolstoi was appointed union leader by the commission of workers, and like their leader, he rebelled against...
A: like their leader
B: as their leader does
C: as their union leader

Now in this sentence, the context indicates that Tolstoi is actually the Union leader. So the intended meaning here is "FUNCTION" or "ROLE". Thus, the correct usage is "as their leader". Choice C is correct here. A and B are not correct.

TAKE-AWAY
Always understand the intended meaning of the sentence.
Comparison can be expressed in two ways - Like + NOUN and As + CLAUSE
If the meaning is "in the role of", then you must use the expression - AS + NOUN

Hope this helps

Payal