—How often do I need to feed the birds?

—They ________ food every day，or they will be hungry.

A．must give

B．mustn’t give

C．must be given

D．mustn’t be given

下列纯物质溶于水时，未克服共价键的是（　　）

①CH₃CH₂OH②NaHSO₄③HNO₃④NaOH．

A．①③

B．②④

C．①④

D．②③