Long story cut short-
Contrast in A is one intended.
Contrast, in E is not indended.
How?
A) effects( in general)
but effects (in general).
E) effects( in general)
but effects ( not in general now).
In A, the particularity aspect is in non-essential modifier.
However, in E, the general aspect is in non essential modifier.
A)Discussion of greenhouse effects has usually focused on whether the Earth would warm and by how much, but climatologists have indicated all along that the most obvious effects, and those that would have the largest impact on people, would be extremes of temperature, precipitation, and storminess.
E)Discussion of greenhouse effects has usually focused on whether the Earth would warm and by how much, but climatologists have indicated all along that extremes of temperature, precipitation, and storminess, which are the most obvious effects, are those(effects) to impact the most on people
in A) it is an orange vs an orange,
whereas in E) it is an orange vs a lemon.
Moreover, E also has an unidiomatic issue :
extremes are effects to impact the most on people .
people was supposed to be the direct object of
to impact.
We, however, have
on people(prep phrase) and not
people.
Making it
people will not help either because in that case
most will end up modifying people( meaning change).
Following explanation is not necessary to get to the correct answer choice.
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To get some specific job done, GMAC has a correspondind tool that get that specific job done. For instance,
for causation, we have because, because of, so, thus, therefore, since, as a result, consequently, resultantly etc.
Double causation is incorrect.( Because..., so)or ( because..., thus etc).
Similarly, no indication of cause and effect situation where one mandates is incorrect.
for
contemporaneous actions, we have ,ing ing(w/o comma), simultaneously, as(clause), when, while Ving
Similarly, no indication of contemporaneous situation where one mandates is incorrect.
for
contrast, we have but, yet, however, although, whereas, while(clause) etc. All these tools essentially perform the task of contrast although there are little nuances that make one much suitable for any specific situation, for instance although vs whereas, i believe, is a genuine split/ distinction. Similarly
while clause is much suitable for contrast + contemporaneous situation. However, all these tools necessarily indicate some form of contrast in core.
Double contrast in GMAT SC (for a single contast) is incorrect if both contrasting expressions are conjunctions.(
but yet is the most popular one).
However, reinforcing is allowed. for instance but instead, but rather ( conjunction( that shows contrast) + adverb ( that shows contrast and thus reinforces the contrasting aspect of situation)) is perfectly acceptable.
Similarly, no indication of contrasting situation where one mandates is incorrect.
Please note that omitting the contrasting expression( such as but, however) is absolutely acceptable if the contrast still can be communicated properly; it is possible to omit contrasting word/phrase/expression and still maintain contrast in core [A].
Therefore, if there is a contrasting expression in a sentence, then there must be a contrasting sitution in hand.
PS:
[A] we do have official SCs in which there was contrast but no contrasting word/phrase/expression