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

调节抽汽供热式汽轮机在额定参数范围内的调节过程中，当（）负荷改变时，不会影响（）负荷的供应，相反也是成立的。