— Will you join us in the game?    

— Thank you, ____    [      ]

A. but why not?    

B. but I'd rather not.    

C. and l won't.    

D. and I'll join.

烧开水要从锅的底部加热，冷冻温水要把冰块放在上面，为什么？试用所学知识解释之。