You can only be sure of ____ you have at present; you cannot be sure of something ____ you might get in the future.

A．that, what

B．what, /

C．what, which

D．/ , that

过程气管线伴热主要作用正确的是（）。

A、增加管内流动速度

B、增加管内气化率

C、防止过程气中水蒸汽冷凝

D、防止过程气中硫蒸汽的冷却凝固