LSAT-19-3-17

17.   A new silencing device for domestic appliances operates by producing sound waves that cancel out the sound waves produced by the appliance. The device, unlike conventional silencers, actively eliminates the noise the appliance makes, and for that reason vacuum cleaners designed to incorporate the new device will operate with much lower electricity consumption than conventional vacuum cleaners.

Which one of the following, if true, most helps to explain why the new silencing device will make lower electricity consumption possible?

(A) Designers of vacuum cleaner motors typically have to compromise the motors’ efficiency in order to reduce noise production.

(B) The device runs on electricity drawn from the appliance’s main power supply.

(C) Conventional vacuum cleaners often use spinning brushes to loosen dirt in addition to using suction to remove dirt.

(D) Governmental standards for such domestic appliances as vacuum cleaners allow higher electricity consumption when vacuum cleaners are quieter.（A）

(E) The need to incorporate silencers in conventional vacuum cleaners makes them heavier and less mobile than they might otherwise be.

Why A is the answer? 为设么 lower efficiency means higher consumption of electricity? efficiency 也可以是指单位时间完成的吸尘工作啊，为什么一定是所消耗的电力？这不是自己设定一个假设么？如果这样的话，d也可以是对的，因为也可以假设因为政府对吸尘器的耗电标准高，原来的吸尘器就设计成高耗电。

有用功率（用去吸尘部分）=效率（efficiency）*总功率（耗电部分，或铭牌上标的功率）。在相同有用功率下，效率高了，总功率（耗电部分，或铭牌上标的功率）你说低不低。即耗电低了。D是解释用电大的现象（ higher electricity consumption when vacuum cleaners are quieter），原文是用电低的现象