Great as Newton was, many of his ideas_____today and are being modified by the work of scientists of our time.

A.are to challenge                 B.have challenged

C.are challenging               D.have been challenged

Great as Newton was, many of his ideas _______ today and are being modified by the work of scientists of our time.

Great as Newton was, many of his ideas_____today and are being modified by the work of scientists of our time.

1. A.
   are to challenge
2. B.
   have challenged
3. C.
   are challenging
4. D.
   have been challenged

Great as Newton was, many of his ideas _____today and are being modified by the work of scientists of our time. 

A. are to challenge      B. may be challenged     C. have been challenged     D. are challenged

Great as Newton was, many of his ideas _____today and are being modified by the work of scientists of our time.