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

问题补充：

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

A. closely

B. closed

C. closing

D. close

答案：

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”。