No sooner _____ themselves in their seats in the theatre _______ the curtain went up.

A．they have settled; before

B．had they settled; than

C．have they settled; when

D．they had settled;than

下列含有非极性键的共价化合物是[ ]

A.Cl₂

B.Na₂O₂

C.C₂H₂

D.CO₂