The doctor must be 24 hours a day 7 days a week in the boarding school. A. on callB. by

问题补充：

The doctor must be 24 hours a day, 7 days a week in the boarding school.

A. on call

B. by hand

C. in sight

D. on spot

答案：

(答案→)A. on call

B. by hand

C. in sight

D. on spot

解析：考查介词短语：A. on call随叫随到，待命，B. by hand在手边，C. in sight在视野以内，D. on spot在出事地点，句意：在寄宿学校，这个医生必须一星期7天一天24小时待命。选A。