While he was investigating ways to improve the telephone,Newton made ________ discovery which completely changed ________ man's understanding of color．
A.a;/B.a;theC./;theD.the;a在线课程A
解析：
discovery发现，前加不定冠词表泛指；
第二空后man一词的单数形式且前无冠词，意为“人类”。