变量x，y满足约束条件y≤xx+y≥2y≥3x-6，则目标函数z=2x+y的最小

Not until his mother came back_______.

A．he began to do his homework

B．had he began to do his homework

C．he had begun to do his homework

D．did he begin to do his homework

关于燃烧的叙述正确的是（　　）

A．燃烧一定是化学反应

B．燃烧一定有发光放热现象

C．有发光放热的变化一定是燃烧

D．凡燃烧的物质，一定有火焰产生

过重的课业负担严重损害儿童少年身心健康。《教育规划纲要》提出要把减负落实到中小学教育全过程，促进学生生动活泼学习、健康快乐成长。率先实现（）减负。

纳西族的基本情况是怎样的

湿温湿热化燥，伤络便血，症见便下鲜血，伴灼热烦躁，舌质红绛，治当（）

A.犀角地黄汤

B.黄土汤

C.犀角地黄汤加地榆炭、侧柏炭、止血草、茜草炭等

D.加减玉女煎

E.黄连解毒汤