The government has taken some measures to solve the shortage of electricity, but it may be some time _______ the situation improves.  

A．since

B．when

C．unless

D．before

下列反应中属于中和反应的是（　　）

A．Na₂CO₃+2HCl=2NaCl+H₂O+CO₂↑

B．2NaOH+H₂SO₄=Na₂SO₄+2H₂O

C．SO₂+2NaOH=Na₂SO₃+H₂O

D．Zn+CuSO₄=ZnSO₄+Cu