The boy seized a stick and ___ the dog, but it ran away.

A．beat

B．struck at

C．hit

D．knock at

下列反应不能通过一步反应制得的是

①SiO₂→Na₂SiO₃　②CuSO₄→CuCl₂③SiO₂→H₂SiO₃ ④CuO→Cu(OH)₂　

⑤Na₂O→Na₂SO₄

A．①②

B．③④

C．②③④

D．②③④⑤