关于正比例函数y=-2x，下列结论中正确的是（ ）A.图象过点（-1，-2）B

用于初步鉴别肠道致病菌和非肠道致病菌的试验是（）

A.蔗糖发酵试验

B.乳糖发酵试验

C.葡萄糖发酵试验

D.甲基红反应试验

E.枸橼酸利用试验

中观区位往往对区域城市间土地的级差地租和地价水平产生决定性的作用。（）

旅客自愿退国际客票，退款应以（）进行计算。

A、付款地国家货币

B、办理退款地国家货币

C、始发地国家货币

D、以上都不对

Children going to school must have a chance to ______ excess physical energy; children even more than adults require and enjoy physical exercise and games of all kinds.

A．work off

B．let off

C．play off

D．put off

These plants are rather difficult（）

A.to look after

B.to be looked after

C.being looked after

D.looking after