4. The gyrfalcon, an arctic bird of prey, has survived a close brush with extinction; its numbers are now five times greater ... were when the use of DDT was sharply restricted in the early 1970's.
(A) its numbers are now five times greater than what they were when
(B) its numbers now fivefold what they were when
(C) its numbers now five times more than when
(D) now with fivefold the numbers it had when
(E) now with its numbers five greater since

All sorts errors are at play, actually, which is what makes the GMAT so tricky.

There are parallelism issues ("its numbers now" should be matched with a similar grammatical piece for "its numbers thirty years ago").

There are idiomatic issues. You can say that "Tom's collection increased fivefold," but you cannot say "Tom's collection is fivefold the number it once was."

Along these lines, greater cannot be used as it stands in (E).

There are also comparison issues (which dovetail nicely with parallelism issues). If you say some number is five times greater than it was 30 years ago, you need the word "than."

If you'd prefer specifics as to why each answer is wrong, I'm happy to volunteer what I know, but it seemed as if you were asking for more general info about this question.

I felt that all the other choices would have needed a comma (just before the underlined part)... so I right away chose A.

Is this reasoning right?