A:H2SO4→ SO2
B:Zn→ Zn2+
C:H2→H2O
D:Cl2→Cl-
如图示,要使L1和L2组成串联电路,应闭合的开关是____;要使L1和L2组成并联电路,应闭合的开关是____;若同时闭合开关____,则会造成电源短路.