去括号：a-（b-c+d）=______．
a-（b-c+d）=a-b+c-d．
如图所示，要使灯L1和灯L2串联在电路中，应闭合的开关是______；要使灯L1和灯L2并联在电路中，应闭合的开关是______；开关______不能同时闭合，否则会发生短路．
The seaside here draws a lot of tourists every summer. Warm sunshine and soft sands make ____ it is.[ ]
A. what
B. which
C. how
D. where
