What____these boxes______.A.did,make of B.were,making of C.are,made of D.was,made of

What____these boxes______.
A.did,make of B.were,making of C.are,made of D.was,made of

C
be made of 由……制成
boxes说明是复数,排除D,要用be动词不用A,语态是被动,所以综合就是C

C

C这些盒子是什么做的