We plan to reach the North Pole in mid-July, and by then we for six weeks.
A. are walking
B. have been walking
C. will be walking
D. will have been walking
网友回答
A. are walking
B. have been walking
C. will be walking
(答案→)D. will have been walking
解析：句中by then短语表示一个将来的时间状语，所以谓语用将来完成时，因此选D。