The ___boy was last seen ___near the river.

The ___boy was last seen ___near the river.
A missing,playing B missing,play
为什么后面一个空填play、这不是动词原形么?
__ a reply,he decided to write again,
A Not receiving B Not having receiving
为什么选B不选A、有什么区别
I begin ___the meaning,which begins__.
A understanding,to be clear
B to understand,to be clear
C understanding,being clear
D to understand,being clear
为什么是选B、这句话怎么翻译