单选题 So small ______ that the most powerful microscopes cannot detect them.
A. are these particles B. were these particles
C. these particles are D. these particles were

【正确答案】 A
【答案解析】so+(形容词/副词)结构放在句首，后面的部分要用倒装形式。
译文：这些微粒如此之小，以至于用最高倍显微镜都无法看到它们。