a number of students _ the great wall already.

have been

have been to

have been

首先 a numble of 是表示大量。用负数
其次，already要和现完连用

have been
a number of students 很多同学 是复数
因为有already 所以用完成时

have been

主语是复数
already是完成时的标志，不能用went

(the前少了个to)
选第1个
因为有already 所以用完成时
前面是a number of 所以用have
如果a改为the 就用has