The city was difficult was defended by an army of 6 000 men.A. to be taken; whichB. to

问题补充：

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

A. to be taken; which

B. to take; that

C. to take; which

D. to be taken; that

答案：

A. to be taken; which

B. to take; that

(答案→)C. to take; which

D. to be taken; that

解析：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。