hi
because 'since' is used in the sceond clause, we should have present perfect usage. [has/have]

'the number' is singular subject. hence we should use 'has' instead of 'have'

parallelism need to be maintained..."has grown steadily" and "has reached record high"

A) uses have instead of has
B) no use of perfect tense
C) subject has changed from "number" to "students" in the second clause
D) correct
E) subject changed, usage of have

ans D

any corrections?

Thanks Ranji.

But could you explain me when to use ":" instead "," or ";"?

: - the sentence before the : will be an independant clause
; - ; separates two independant clauses
, - is basically used to set up a modifier

any corrections/additions on my points?