求助，这题highlight部分怎么理解？思路是什么？prep里这种题出了4道，一句话没明白，就影响4道题。9天就考试了，急！先谢谢了！！
118.
12240-!-item-!-187;#058&008164 For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean. What was the mean score for the examination?
(A) 74 (B) 76 (C) 78 (D) 80 (E) 82 | 
