His flat is the floor in a building on a busy streetA. in nine B. in ninth C. on nineD

问题补充：

His flat is the floor in a building on a busy street．

A. in, nine

B. in, ninth

C. on, nine

D. on, ninth

答案：

A. in, nine

B. in, ninth

C. on, nine

(答案→)D. on, ninth

解析：住在几楼要用介词on,几楼要用序数词。