但是在测试开发时期修正一个薄弱且错误的用例十分困难而且成本很高，尤其是当使用一个基于瀑式开发方法的时候。
But correcting a weak or erroneous use case at test development time can be difficult and costly, especially when using a waterfall-based development method.
但是，因为他们对平常物质只有非常微弱的影响，对之进行观测十分困难。
But, because they interact only very weakly with normal matter, detecting them is difficult.
但是，刚毕业的大学生们要找专业对口的工作就十分困难。
However, young college graduates are finding it more difficult than ever to land jobs in their field.
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