531.New techniques in thermal-scanning photography, a process that records radiation from surface areas, makes it possible to study the effects of calefaction, or warming, of a river in greater detail than ever before.
(A) makes it possible to study the effects of calefaction, or warming, of a river in greater detail than ever before
(B) make it possible to study, in greater detail, the effects of calefaction, or warming, of a river than ever before
(C) have made it possible to study in greater detail than ever before the effects of calefaction, or warming, of a river
(D) make possible the study of the effects of calefaction, or warming, of a river in greater detail than it ever was before
(E) has made it more possible than ever before to study in greater detail the effects of calefaction, or warming, of a river

Key is C . have made it possible to study in greater detail than ever before the effects of calefaction, or warming, of a river

答案是C没有问题.我不太明白made后面的it做什么成分,指代什么?

it = to study in greater detail than ever before the effects of calefaction, or warming, of a viver.

to study in greater detail than ever before the effects of calefaction, or warming, of a viver 太长, 后置, 用 it 指代