---- I am not able to solve some practical problems using Newton’s First Law of Motion.

----That’s just _______ most of the students are puzzled easily.

A．what

B．why

C．how

D．where

关于脊髓灰质炎病毒减毒活疫苗描述不正确的是

A．疫苗株不耐热，储存和运输中必须冷藏
B．免疫缺陷者必须慎用
C．疫苗株有回复毒力的危险
D．口服一次即可，血清抗体阳转率达100%
E．疫苗病毒随粪便排出，扩大了免疫面