It is_______ that I can still remember_______ was discussed at the meeting.

A．sure; when

B．sure; what

C．certain; when

D．certain; what

釜式重沸器的特点叙述正确的是（）

A.汽化率低

B.本身有蒸汽空间

C.相当于一块实际塔板

D.金属耗量低