A number of experts ________ invited, but the number of the experts who attended the conference ________ only 20．

A.were; was

B.was; were

C.was; was

D.were; were

答案及解析： A

解析：

a number of+名词复数，“许多”，谓语动词用复数；the number of…的数量，谓语动词用单数．