[Grammar] more than tripled

[Hồ Quang Trung]

__________ in universities around the nation has more than tripled compared to this time last year.

A. enroll
B. enrolls
C. enrolling
D. enrollment

Key: D

I understand the question and agree with the key provided. However, there is one problem I still cannot figure out.

has more than tripled => has increased more than tripled

Is this right? Or has more than tripled is enough. If it is, please explain.

 

[Tarheel]

Has more than tripled - has increased more than threefold