在一定温度和压强下，1体积的A₂气体和3体积B₂气体化合成2体积的C气体，则C的化学式为(    )

A.AB₃               B.AB               C.A₃B              .A₂B₃

— I'll tell Mary about her new job tomorrow.

— You ______ her last week. [ ]

A. ought to tell

B. would have told

C. must tell

D. should have told