当地村民在此开采煤炭的历史在300年以上。废旧煤洞的存在成为了工程的一大隐患，煤洞问题成为工程的主要地质问题之一。
Local villagers are mining for more than 300 years, so the abandoned and old coal mines are one of main geologic problems.
鸽子洞原理是分析数学组合的有力工具。但是其概念简单，可以通过以下具体的问题加以阐述。
The pigeonhole principle is a powerful tool used in combinatorial math. But the idea is simple and can be explained by the following peculiar problem.
问题之一在于高尔夫的昂贵开支：18洞的平均费用为40美元。
One problem is that golf is expensive: the median fee for 18 holes is $40.
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