Strange about C. I have seen writers use "not only X but Y" in the real world but it's rare - and I have never seen the test do that.

E technically breaks the idiom too, though. It says

"were not only able to... but they were also able to..."

Correct usage should be "were not only X but also Y"

If I had had to choose on this one, I'd probably have gone with E also. You should email them and ask! (And let us know if they reply!)

Since I first posted (wow - almost 2 years ago now!), I have seen the test use the "not only X but Y" setup (omitting the "also"), so the GMAT writers apparently think this idiom is an acceptable alternative. As such, I would've been wrong in picking E. Both C and E contain acceptable idioms, but E breaks parallelism and C doesn't... so C wins.

You've got it!

In a double idiom (not only X but Y), the part before the idiom starts (so before "not only") must apply equally to both X and Y. We've got:

Scientists were not only X (able to...) but Y (they were also...).

Scientists were able to blah blah blah. That's okay.
Scientists were they were also able to. That's not okay.