Under the ask an English

the missing boy was last seen _____near the river.
A.had played B.playing C.to play D.played

The answer is B, then C why not? though, said the boy was the last time When people see that the river is playing, but there is a syntax sb was / were seen to do, why this title is not available sb was / were seen to do

This is a adverbial with the past, when completed, C is carried out in the past, should not the same D you