The doctors are doing all that they but the old man is still not breathing properly.A. s

问题补充：

The doctors are doing all that they , but the old man is still not breathing properly.

A. shall

B. need

C. may

D. can

答案：

A. shall

B. need

C. may

(答案→)D. can

解析：考查情态动词：A. shall 将会，B. need需要C. may可以，可能D. can能够，句意：医生在做他们能做的一切，但老人还是不能正常呼吸。选D。