下列不属于细胞因子的是（）

A.淋巴毒素

B.过敏毒素

C.IL-1

D.CSF

E.IFN

----- Mary looks very upset. She _______ failed to pass the exam.

----- I guess so. It's very difficult after all.

A  must have   B could have   C should have  D might have