如图所示电路.
(1)当开关S1,S3断开,开关S2闭合时,L1和L2组成______电路;
(2)当开关S3闭合,S2和S1断开时,L1是______路,L2是______路;
(3)当开关S3断开,S1和S2闭合时,L1是______路,L2是______路.
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