能采用阳离子、阴离子与自由基聚合的单体是（）
参考答案：St
一元二次方程x2=4的常数项是______．
The old building,behind_____ was a famous church,was _____ we used to work.[ ]
A. that;the place
B. it;the place
C. which;where
D. what;where
