The number of people invited fifty but a number of them absent for different reasons.A

问题补充：

The number of people invited fifty, but a number of them absent for different reasons.

A. was, was

B. were, were

Cwere, was

D. was, were

答案：

A. was, was

B. were, were

C. were, was

(答案→)D. was, were

解析：考查：The number of +复数名词作主语，……的数量，谓语动词用单数；a number of+复数名词作主语，许多，谓语动词用复数形式。所以选D。