假设某证券上升至20元处遇到阻力回落，至17元重新获得支撑，反弹至20元，此后一路下

一个带正电的点电荷，电量q=2.0×10^-9C，在静电场中由a点移至b点，在这个过程中，除电场力外，其它力做的功为6.0×10^-5J，质点的动能增加了8.0×10^-5J，求a、b两点间的电势差U_ab．

插胃管时如病人出现恶心，应暂停片刻，嘱病人做深呼吸或吞咽动作。

论述降低火电厂汽水损失的途径？

利用领带捆绑时，将敌双臂于背后交叉，两（）重叠，用领带在其腕关节处缠绕一圈，打一死结即可。

Purebred cows native to Mongolia produce, on average, 400 liters of milk per year, if Mongolian cattle are crossbred with European breeds, the crossbred cows can produce, on average, 2,700 liters per year. An international agency plans to increase the profitability of Mongolia’s dairy sector by encouraging widespread crossbreeding of native Mongolian cattle with European breeds.
Which of the following, if true, casts the most serious doubt on the viability of the agency’s plan

A．Not all European breeds of cattle can be successfully bred with native Mongolian cattle.

B．Many young Mongolians now regard cattle raising as a low-status occupation because it is less lucrative than other endeavors open to them.

C．Mongolia’s terrain is suitable for grazing native herds but not for growing the fodder needed to keep crossbred animals healthy.

D．Cowhide and leather products, not milk, make up the bulk of Mongolia’s animal product exports to Europe.

E．(E) Many European breeds of cattle attain average milk production levels exceeding 2,700 liters.