---I’m going to Vietnam?---How long you in Vietnam?A. are; stayed　　B. are; stayingC. have

问题补充：

---I’m going to Vietnam?

---How long you in Vietnam?

A. are; stayed

B. are; staying

C. have; stayed

D. did; stay

答案：

A. are; stayed

B. are; staying

C. have; stayed

D. did; stay

解析：句意：——我要去越南？你要在越南呆多久？根据语境，表将来完成，How long多久，表示时间段，经常和完成时态连用，此处用现在完成时表示将来完成。故选C。