If you don't know the answer to something,  you say: "_______" [ ]

A. Sorry, I don't understand.

B. I'm sorry. I don't know.

C. Can you speak English?

D. Excuse me. How do you say that in English?

We’ve a lot of things to do. But _____ we must get some sleep.

A．at first

B．at once

C．first of all

D．the first thing