what is right usage and please explain

[new2grammar]

The growing number of vistors ---------- the footpaths.
a. is damaging b. are damaging


Jane --------- just a few minutes ago.
a. left b. had left
why using had left is wrong here?


--------- Robert lately?
(a) Did you see
(b) Have you seen
I get confuse which is right answer and how to select

[SirGod]

*Not a teacher

The growing number of visitors is damaging the footpaths. - singular, because the subject is "the growing number", not "visitors"

Jane left just a few minutes ago. - the time is specified: a few minutes ago. We use past simple when the time is specified.

Have you seen Robert lately? - we use perfect tenses with lately