急问!

我正在申请一个PhD Vacancy, 上面说的是if you have interest, please e-mail a detailed CV as well as a letter of application to ...,   请问 a letter of application 是什么样的一个letter呢?, 需要写哪些关于申请人的东西呢?如果申请很多vacancies的话, the content of the application letter是不是要有所变化呢?

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