—Can I call you back at two o’clock this afternoon? —I’m sorry but by then I to Beijing.

问题补充：

—Can I call you back at two o’clock this afternoon?

—I’m sorry, but by then I to Beijing. How about five?

A. fly

B. will fly

C. will be flying

D. am flying

答案：

A. fly

B. will fly

(答案→)C. will be flying

D. am flying

解析：考查动词时态。根据问句中的two o’clock和应答句中的by then可知所填词表示的动作在当时正在进行，用将来进行时，选C。