硬膜外阻滞完善的时间需（）
A.3～5min
B.8～10min
C.12～20min
D.25～30min
E.40min以上
参考答案：C
Scientists are confident that they can make lasers which can ____ missiles.
A．knock down
B．knock out
C．knock into
D．knock off
能经接触感染的节肢动物有：（）、（）、（）。
