请教一个sc，来自刘振民语法

E9—13 Since the movie was released seventeen UFOs have been sighted in the state，which
is more than had been sighted in the past ten years together．
(A)which is more than had been sighted
(B)more than had been sighted
(C)more than they had sighted
(D)more than had reported sightings
(E)which is more than had reported sightings

答案是B。请问这个从句是什么从句？主语为何能省略呢？

Since the movie was released seventeen UFOs have been sighted in the state

=Seventeen UFOs have been sighted in the state since the movie was released.

所以主语没有省略, 后面的more than 是同位语,补充说明17个UFO 比以前看到的要多