Having a trip abroad is certainly good for the old couple, but it remains ______ whether they will enjoy it.[      ]

A. to see

B. to be seen

C. seeing

D. seen

在连续流动的氧气系统中，输往氧气面罩的氧气量怎样控制（）

A、管路活门

B、定量孔

C、减压活门

D、驾驶员的调节器