It has been proved eating vegetables in childhood helps to protect you against serious illnesses in later life.
A. if
B. because
C. when
D. that
网友回答
A. if
B. because
C. when
(答案→)D. that
解析：此题考查that引导的主语从句的用法。句中的it是形式主语，真正的主语是eating vegetables in …life,所以要that引导这个主语从句，而that不作任何成分，也没有意义，故答案是D项。