A:只闭合开关S1
B:只闭合开关S2和S3
C:只闭合开关S2
D:开关S1、S2、S3都闭合
①鸦片战争
②第二次鸦片战争
③甲午中日战争
④八国联军侵华战争
若干量x,y满足约束条件x+y≥1y-x≤1x≤1,则z=2x-y的最小值为()
—Linda, I called you this morning, but nobody answered the phone.
—I'm sorry, I________ football with my friends then.