There are two blackboards the wall and two doors the wall.A. on onB. on in C. in onD.

问题补充：

There are two blackboards the wall and two doors the wall.

A. on, on

B. on , in

C. in , on

D. in ,in

答案：

A. on, on

(答案→)B. on , in

C. in , on

D. in ,in

解析：句意：墙上有两块黑板，两个门。墙上的黑板、图画、地图等都是墙外的东西，强调它们在墙上，用on；墙上的门、窗、洞等属于墙以内的东西，用in，故选B。