题目

---- My sister is a typist at a foreign firm.    ---- I know this sort of work _____ skill and speed.   A. asks for     B. calls for  C. looks for         D. waits for   答案：B如图所示，直线B为电源的U-I图线，直线A为电阻R的U-I图线，该电源和该电阻组成闭合电路时，电源输出功率和电源的效率分别是( 　)A．电源的输出功率为2WB．电源的输出功率为4WC．电源效率约为33.3%D．电源效率约为66.7%