问题补充：
-- A medical team was sent to Japan by air five hours ago.
---So they have arrived by now. The flight usually takes four hours or so.
A. might
B. could
C. need
D. must
答案：
A. might
B. could
C. need
(答案→)D. must
解析：句意：一个医疗队在5个小时前已经派往日本。所以他们到现在为止肯定已经达到了，航班通常是花费4个小时的。must have done肯定已经做了某事，表示对过去的肯定推测。故选D
