The GRE, which was first administered in 1949, is four years older than is the GMAT.
(A) which was first administered in 1949, is four years older than is
(B) first administered in 1949, is four years older than
(C) which was first administered in 1949, is four years older than
(D) first administered in 1949, is four years older as is
(E) first administered in 1949, is four years older than is


Is it necessary to have older than 'is'....why is not B correct?

With all due respect to GyanOne, I disagree.

I believe this question is fundamentally flawed. In my reading, (B) is 100% correct. The subject is not "the GRE's age", but rather just "the GRE". If we drop the modifying clause between the commas, (B) becomes:
(B) The GRE is four years older than the GMAT.
That is a classic correct comparison --- noun to noun comparison. Adding the modifier doesn't change anything. (B) is 100% correct. In fact, (A) & (C) are also grammatically correct, although a bit wordier.

What is the source of this question? Keep in mind that not all GMAT SC practice questions are created equal. Some are excellent --- for example, MGMAT consistent produces high quality questions. Many, even from companies with reputable names, are absolutely atrocious. If I had to give the question writer a grade for this question, I would give an F. This is an abysmal failure of a question.

In a true GMAT SC question or a high quality practice question, there are clear grammatical errors separating most of the incorrect choices from the correct choice, and the wordiness/concision thing is usually not used as a split for more than one answer.

Here's a high quality question, a practice SC question dealing with comparisons:
http://gmat.magoosh.com/questions/3229
When you submit your answer, the following page will have a complete video explanation. Each one of Magoosh's 700+ GMAT practice questions has its own video explanation, for accelerated learning.

Let me know if anyone reading this has any further questions.
Mike
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Mike McGarry
Magoosh Test Prep