Our school had about 20 notebook computers but only one-forth used regularly. No

 Our school had about 20 notebook computers but only one-forth_____used regularly. Now we have 50 working all day long.  
     A. is                B. are                   C. was      D.were

试题答案

【解析】D 考查主谓语的一致性原则和省略句式。依据one-forth （of the computers）可知。