Sentence Correction

If birds did not have wings, they would not have been able to fly

A. If birds did not have wings, they would not have been
B. If birds had no wings, they would not have been
C. Unless birds had wings, they would not be
D. If birds did not have wings, they would not be
E. Unless birds had wings, they would not have been

The accurate answer is c.

I am confused and i do not understand why D is not the right option to consider?