46页21.Sam ，have you ------------before？Never。but I watched somebody else ----------。

b。ski,skiing,skied
滑雪，
完成时用过去分词,
进行时表状态，意为看到某人正在做XX。

C
have you...现在完成时，用ski的过去时态，
I watched somebody else。。。是说看见某人在滑雪
选项b的skiing是滑雪的名词，在这里看见某人做某事用ing形式，所以选C

have you skied before?

never, but i HAVE watched somebody else skiing.

两个have才能形成问答的形式。你付钱跟我学英文把。