Bare Infinitive after see, hear, watch and notice

According to the grammar book I have at hand, we should use Bare Infinitive after the verbs see, hear, watch and notice. However,

The police arrested him as he was seen _____ the bank at the time of the robbery.

a) enter
b) to enter
c) to entering
d) entering in

The test suggests that answer “b” is correct. How about Bare Infinitive? Can anyone explain?

It is the only grammatically correct answer.

yes, but why?

Read the context of the test question … it fits … none of the others do!

Hi KArmine,
We normally use the bare infinitive or -ing after these verbs in active-voice sentences:
I saw the boys PLAY/PLAYING football.
But in passive voice,infinitive or -ing should be used:
The boys were seen TO PLAY/PLAYING football.
Also,“to enter” doesn’t need any prepositions;therefore,ENTERING IN is wrong.
Hope this helps!

Yes, this really helps! Thanks a lot!!!