I think its A

In order to paint a rough surface, it is necessary to do X and Y before applying the paint. X and Y needs to be parallel.

X(to sand) and Y(to apply primer) are parallel.

Here "it" properly refers to "rough surface". "It" cannot refer to the verb "paint" ("paint" acts as a verb here).

There are only 2things X and Y to be done before applying the paint.

B, C and E changes the meaning. They list 3 things to be done..
D is wordy.

OA?

Hi Rohan,
In order to evenly and effectively paint a rough surface, it is necessary to sand it and apply primer before applying the paint.

Carefully look at this piece of the sentence "apply primer before applying the paint." Had the it refered to paint there is no meaning in saying before applying paint.

So it should refer to the surface and has a proper referent.

This is my tip not sure if everybody agrees to this.

So A

IMO A

Approach :

In the non-underlined part, the word NECESSARY indicates that something has to be done before something. I think, the word BEFORE is required to compliment the word NECESSARY here.
Suppose you need to advise someone that "marking before cutting a cloth is necessary to cut the cloth to correct dimensions"
It is necessary to stress on what exactly is required. it is required to "mark it before cutting" so that you can cut it to proper dimensions:

by this i shortlisted A and D,
Please note : 'IT' is not ambiguous in any of the sentences. There is only one noun "surface" to which 'it' clearly refers to.

Now,between A and D, D is wordy. When you could just do with a single word "it" with no ambiguity, why use " the surface".

I am no expert, but when I initially saw this question and saw option (A), I felt there has to be better one. Turns out, all the remaining four were worse than (A) and here is why:

(B) 2 "and"s at the end is not preferred.
(C)The opposite problem here - no "and"s - with three clauses, a connector is needed.
(D)sand the surface and apply primer before you apply paint (sentence should stop here, because apply primer worked without "surface" - either use the word "surface", or the pronoun or nothing after introducing it in the first clause)
(E) If "surface" is used in the second clause, "it" cannot be used in the third clause.

rohangupta83 wrote:well, we've got quite a few answers

OA - A

but I still don't understand why it is A

can an expert help us?

What is the source for this question?

In order to evenly and effectively paint a rough surface, it is necessary to sand it and apply primer before applying the paint.
(A) it and apply primer before applying the paint
(B) it and prime it and paint it
(C) it, then prime it, then paint it
(D) the surface and apply primer before you
apply paint to the surface
(E) the surface, apply primer to the surface and then paint it

Form the above discussions we know that the irst "It" is a placeholder.
However is'nt the second "it" ambiguous?What is it referring to??The paint or the surface??Please clarify.
B,C>>eliminated
D>>"You">>Eliminate.
Left with A and E.IOs'nt E more clearer as its specifying "The surface" and leaves no room for ambiguity??

In order to evenly and effectively paint a rough surface, it is necessary to sand it and apply primer before applying the paint.
(A) it and apply primer before applying the paint
(B) it and prime it and paint it
(C) it, then prime it, then paint it
(D) the surface and apply primer before you
apply paint to the surface
(E) the surface, apply primer to the surface and then paint it

Lets analyze.
B and C are eliminated directly as they are using Prime.
For D and E the word "Surface" is repetative.
Only "A" ensures that Surface is not repetative and the pronoun points to SUrface.

moneyman wrote:"It" refers to surface and the sentence is structurally parallel

I did not choose E because it uses surface twice and it sounds redundant

Yeah, I went with A for the same reasons. There was nothing else for "it" to be referring to other than the "surface," and the rest was worded fine.