--- Sorry to say that I can’t play football with you this afternoon, for I’ll have to prepare for the coming examination.

--- _____. Have some fun!

A．Don’t be a wet blanket

B．It’s not my cup of tea

C．It doesn’t matter

D．We’re all ears

通常混凝土中外加材料掺加量占水泥重量（）的称为外加剂。