带状疱疹的潜伏期为（）
A.难以确定
B.12～21天
C.1～3天
D.5～7天
E.7～14天
参考答案：A
The second book _______ you are reading is written by an American novelist.
A．不填
B．whose
C．which
D．what
旋启式止回阀密封性差些，只能安装在水平管道上。( )
