The company had about 20 notebook computers but only one-third ________ used regularly．Now we have 60 working all day long．

A.is

B.are

C.was

D.were

答案及解析： D

解析：

本题考查主谓一致及动词的时态，当分数、百分数作主语时，要根据其后面的名词来决定其谓语动词的单复数，此处one-third表示one-third of the notebook computers，因此谓语动词用复数，且句子的时态用了过去时，所以D项正确．