问题 选择题
如图所示为一磁流体发电机示意图,A、B是平行正对的金属板,等离子体(电离的气体,由自由电子和阳离子构成,整体呈电中性)从左侧进入,在t时间内有n个自由电子落在B板上,则关于R中的电流大小及方向判断正确的是(  )

A.I= ,从上向下

B.I= ,从上向下

C.I= ,从下向上

D.I= ,从下向上

答案

答案:A

填空题
单项选择题

If soldiering was for the money, the Special Air Service (SAS) and the Special Boat Service (SBS) would have disintegrated in recent years. Such has been the explosion in private military companies (PMCs) that they employ an estimated 30 000 in Iraq alone -- and no government can match their fat salaries. A young SAS trooper earns about£ 2,000($3 500) a month; on the "circuit", as soldiers call the private world, he could get £15 000. Why would he not

For reasons both warm-hearted and cool-headed. First, for love of regiment and comrades, bonds that tend to be tightest in the most select units. Second, for the operational support, notably field medicine, and the security, including life assurance and pension, that come with the queen’s paltry shilling.

Although there has been no haemorrhaging of special force (SF) fighters to the private sector, there has been enough of a trickle to cause official unease. A memo recently circulated in the Ministry of Defence detailed the loss of 24 SF senior non-commissioned officers to private companies in the past year. All had completed 22 years of service, and so were eligible for a full pension, and near the end of their careers. Yet there is now a shortage of hard-bitten veterans to fill training and other jobs earmarked for them, under a system for retaining them known as "continuance."

America has responded to the problem by throwing cash at it, offering incentives of up to $150 000 to sign new contracts. The Ministry of Defence has found a cheaper ploy. It has spread the story of two British PMC employees, recently killed in Iraq, whose bodies were left rotting in the sun

The word "security" in the second paragraph most probably means()

A. protection

B. document

C. certificate

D. safety