百白破混制剂基础免疫3针的程序为（）
A.3，4，5月龄
B.2，3，4月龄
C.4，5，6月龄
D.5，6，7月龄
参考答案：A
We must keep a positive attitude toward the future ________ we may encounter some difficulties.[ ]
A. however
B. as if
C. even if
D. since
