以下哪项不是小儿肺炎的高危因素（）。
A.病毒感染
B.营养不良
C.先天性心脏病
D.生活环境拥挤
E.免疫缺陷者
参考答案：A
为什么PVC树脂要有型号区别？
There are so many CDs that I don't know ______. [ ]
A. to buy which one
B. which to buy one
C. which one to buy
D. to buy one which
