Question

【题目】LED lighting, even in most developed countries, ________ long enough to know its effects across the human lifespan.

A.has not been usedB.was not used

C.had not been usedD.is not being used

Answer 1

【答案】A

【解析】

考查时态。句意：即使在大多数发达国家，LED照明的使用时间也不够长，不足以了解其在人类寿命中的影响。分析句子可知，LED照明到现在一直还在使用，所以用现在完成时，故选A。