It has been proved _______ eating vegetables in childhood helps to protect you against serious illnesses in later life.

A．if

B．that

C．what

D．when

罩式炉进行松卷脱氮热处理时，保护气氛的含H2量必须（）。

A.＜25%

B.25%～50%

C.＜70%

D.≥75%