____________, they have been struggling against the snowstorm for ten hours.

A．Up to now

B．By the time

C．From then on

D．In the future

下列哪个麻醉药的麻醉作用最强（）。

A.氟烷

B.异氟烷

C.七氟烷

D.恩氟烷

E.甲氧氟烷