--Who is the person talking with you just now? Well if you know his name is David.A.

问题补充：

--Who is the person talking with you just now?

-- Well, if you know, his name is David.

A. may

B. can

C. must

D. shall

答案：

A. may

B. can

(答案→)C. must

D. shall

解析：考查情态动词：A. may 可以，B. can能够， C. must 必须，偏要D. shall将会，句意是;--刚刚和你说话的是谁？--如果你偏要知道，是大卫。选C。