— Would you like to play tennis with me ______ it doesn't rain tomorrow?

— Sure, ______ I am busy. [ ]

A. if; since

B. whether; but

C. if; unless

D. because; until

加热炉对流室炉管采用钉头管可降低（）的传热热阻，以达到提高对流炉管总传热速率的目的。

A.炉管内侧

B.炉管外侧

C.辐射室

D.炉管内油品