It is so cold in Antarctic that the penguins have to stand to each other to keep warm. A.

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A. closely
B. closed
C. closing
(答案→)D. close
解析：本题考查close的用法。close可以作副词，表示“（距离）近地”“紧密地”；closely意为“紧密地，紧紧地”“认真地”“密切地”，通常不与to连用；closed意为“关闭的”。