It’s nearly 7 o’clock. Helen _______ come, but let’s wait for her for another 5 to 7 minutes.

A．couldn’t

B．can’t

C．mustn’t

D．may not

下列化学式能真实表示物质分子组成的是                            （   ）                  

①NaOH  ②SiO₂ ③SO₃   ④He  ⑤P ⑥Na₂O₂

A．②③⑤

B．③⑤

C．③④

D．③④⑤