Question

The city was difficult ________, _________ was defended by an army of 6,000 men.

1. A.
   to be taken; which
2. B.
   to take; that
3. C.
   to take; which
4. D.
   to be taken; that

Answer 1

C
 The city was difficult to take =" It" was difficult to take the city. 只要这两种句型能够互换，不定式就得用主动的形式。