具有根、茎、叶、花、果实、种子六种器官的植物是（ ）

A．被子植物

B．裸子植物

C．蕨类植物

D．苔藓植物

Many of the things that_________ thought useless_________ man today.[ ]

A. used to be; used to serve

B. were used to being; are used to serving

C. were used to be; are used to serve

D. used to be; are used to serve