The film is very interesting .___students have seen it.A.a plenty of B.many of C.a great many

The film is very interesting .___students have seen it.A.a plenty of B.many of C.a great many
如是A的话应该是A plenty of the students have....
B的话应该是 Many of the students have...
C是对的,A great many students have....
为什么A plenty of 、 Many of 要 加 the