Don't you think ___ necessary to learn the text by heart?

Don't you think ___ necessary to learn the text by heart?
A、that
B、this
C、/
D、it
【正确答案】：D
【题目解析】：此句中it是形式宾语，实际的宾语是to learn the text by heart，因为较长后置前面用it代替，necessary是宾补。