18. Modern navigation systems, which are found in most of today’s commercial aircraft, are made with low-power circuitry, which is more susceptible to interference than the vacuum-tube circuitry found in older planes. During landing, navigation systems receive radio signals from the airport to guide the plane to the runway. Recently, one plane with low-power circuitry veered off course during landing, its dials dimming, when a passenger turned on a laptop computer. Clearly, modern aircraft navigation systems are being put at risk by the electronic devices that passengers carry on board, such as cassette players and laptop computers.
Which one of the following, if true, LEAST strengthens the argument above?
A. After the laptop computer was turned off, the plane regained course and its navigation instruments and dials returned to normal.
B. When in use all electronic devices emit electromagnetic radiation, which is known to interfere with circuitry.
C. No problems with navigational equipment or instrument dials have been reported on flights with no passenger-owned electronic devices on board.
D. Significant electromagnetic radiation from portable electronic devices can travel up to eight meters, and some passenger seats on modern aircraft are located within four meters of the navigation systems.
E. Planes were first equipped with low-power circuitry at about the same time portable electronic devices became popular.
费费18题，答案E，解释是无关。我认为无关的话就是完全没有strengthen啊，而不是least strengthen。是不是ETS认为无关就是least strengthen呢？还有，如果选项中有无关项也有削弱项，那least strengthen又该选哪个呢
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