请教一道英文阅度理解题,求讲解,

请教一道英文阅度理解题,求讲解,
原文：My wife (注：a female mole) builds a nest in one of the high molehills that I have constructed above water level - I call it the fortress because it is much larger than the others,but it is possible to escape from it in an emergency.
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The breeding nest is dry during heavy rain because
A.it is below the ground B.it is built like a fortress
C.it has thick walls D.it has good drainage