--- You may refuse my invitation but you must explain why . --- Oh I must ?A. did I

问题补充：

--- You may refuse my invitation , but you must explain why .

--- Oh, I must , ?

A. did I

B. didn’t I

C. must I

D. mustn’t I

答案：

A. did I

B. didn’t I

(答案→)C. must I

D. mustn’t I

解析：本题中用I must构成的一般疑问句以示强调，情态动词must直接提到句首即可，故本题选C。