数学期望的题目！

请看一道概率题，大家做做看。原题是英文，中文是翻的，不太确切，仅供参考。
As part of the underwriting process for insurance, each prospective policyholder is tested for high blood pressure. Let X represent the number of persons tested until the first person with high blood pressure is found. The expected value of X is 12.5. Calculate the probability that the sixth person tested is the first one with high blood pressure.

Answer is one of the following choices
A. 0.000 B. 0.053 C. 0.080 D. 0.316 E. 0.394

作为保单处理流程的一部分，每位预期的客户都被要求进行高血压检查。假设X代表发现第一个高血压的人员时已经检查过的人数（X包括这个人在内），那么X的期望值是12.5，计算X=6的概率。
我作出的答案是0.0503，好象与哪个选项都不同，哪位能帮我算算看。