求助一道OG逻辑题！！！！

翔帅帅

位大神，麻烦帮我看下这道逻辑题，Cheever College offers several online courses via remote computer connection, in addition to traditional classroom-based courses. A study of student performance at Cheever found that, overall, the average student grade for online courses matched that for classroom-based courses. In this calculation of the average grade, course withdrawals were weighted as equivalent to a course failure, and the rate of withdrawal was much lower for students enrolled in classroom-based courses than for students enrolled in online courses. If the statements above are true, which of the following must also be true of Cheever College?
(A) Among students who did not withdraw, students enrolled in online courses got higher grades, on average, than students enrolled in classroom-based courses.
(B) The number of students enrolled per course at the start of the school term is much higher, on average, for the online courses than for the classroom-based courses.
(C) There are no students who take both an online and a classroom-based course in the same school term.
(D) Among Cheever College students with the best grades, a significant majority take online, rather than classroombased, courses.
(E) Courses offered online tend to deal with subject matter that is less challenging than that of classroom-based courses.
答案是选A，我怎么看了原题的解析还是感觉题目有漏洞呢？题目说退课和课程失败是一样的，在计算网上选课和线下课程平均分的时候，平均分应该等于=（课程分数+退课+失败）/课程总同学数，退课和失败同样会拉低平均分。 我的理解 the rate of withdrawal was much lower for students enrolled in classroom-based courses than for students enrolled in online courses意味着网上退课率是大于线下退课率的，确实退课越多就会越拉低平均分，虽然题目说选网课同学的平均分等于选线下课的同学的平均分，但是并不能说明不退课的选网课的同学的平均分就一定比不退课的选线下课的同学的平均分高啊！！因为：解析并没有考虑到线上和线下失败的同学数，比如，用这个公式来看，平均分应该等于=（课程分数+退课+失败）/课程总同学数。虽然线上退课率更高，但是如果线上选课没过的同学少，而先下选课没过的同学多，就会造成一种可能性就是：其实线下的没退课的同学是平均分是高于线上没退课的同学的平均分，只是因为线下失败的人太多了，而线上选课失败的人少，所以才导致线上和线下选课的同学平均分一样的。我不知道我有没有把我的思路解释清楚了，麻烦各位帮我看看，题目说A一定对，但是我觉得A也不一定对啊，因为它没有考虑失败的同学数！大家来帮我看看这道题，我真的要被弄晕了！！！


谢谢各位！麻烦帮我解决下我的困惑！！万分感激！！

zoeklose1

就如你说的，平均分应该等于=（课程分数+失败+退课）/课程总同学数
注意题目说的是the rate of withdrawal was much lower for students enrolled in classroom-based courses than for students enrolled in online courses.  
说明 online退课/总数<classroom退课/总数
就推出 online（课程分数+失败）/总数> classroom(课程分数+失败）/总数
正好是A

你说”虽然线上退课率更高，但是如果线上选课没过的同学少，而先下选课没过的同学多，就会造成一种可能性就是：其实线下的没退课的同学是平均分是高于线上没退课的同学的平均分“就是online（课程分数+失败）/总数< classroom(课程分数+失败）/总数,如果成立的话 online的总平均分就应该<classroom总平均分，而不是像题目说的相同了。