remaining to be solved

[joham]

This question comes from a test paper:

The teacher said there were still so many problems _____, though we had made great progress.

A. remaining to be solved B. remained to solve
C. remaining to solve D. remained to be solved

The given answer is A. I felt the sentence didn't read smoothly. Should we just say 'there were still so many problems to solve/ to be solved' or 'so many problems remained to be solved' instead of the original?

Could I ask native English teachers to help me with this question?
Thank you very much.

 

[Tdol]

Your alternatives work fine, but the original is OK; it's a bit of a cumbersome phrase, but perfectly grammatical.

 

[Shakespeare's brother]

From my point of view I would say the sentence D is the most natural but it definitely needs a 'which':

'still so many problems which remained to be solved'