After the operation Bill has been doing a lot of physical exercise to build up his .A. ab

问题补充：

After the operation, Bill has been doing a lot of physical exercise to build up his .

A. ability

B. strength

C. energy

D. mind

答案：

A. ability

(答案→)B. strength

C. energy

D. mind

解析：考查名词。build up one’s strength增强体质，固定搭配，故选B。