婴幼儿易患呼吸道感染的最主要原因是（）。

A.纤毛运动较差

B.黏膜柔嫩，血管丰富

C.细胞免疫功能低下

D.分泌型IgA低下

E.IgG低下

____you had worked hard, you would have easily passed the final test.

A．When

B．If

C．Thought

D．Since