21. Yale University is home to one of world’s greatest libraries and three famous public museums and galleries.
A. the; the B. the; 不填 C. 不填；the D. 不填；不填
22. He was told that it would be at least three more months he could recover and return to work.
A. when B. before C. since D. that
23. From the teacher’s patience and understanding, I have learnt what a big responsibility is to educate us.
A. this B. that C. it D. one
