求解Magoosh 的一道RC逻辑题

hongshaomj

[size=13.63636302948px]In a freshman biochemistry class at Newton University, the teacher assigns the class only ‘A’s, ‘B’s’ or ‘C’s. The average on the semester final for the class of 2012 was five points lower than that of the class of 2011. Therefore, the percent of students who received ‘C’s was greater in 2012 than in 2011.

[size=13.63636302948px]Which of the following, if true, suggests the conclusion above is not necessarily valid?

[size=13.63636302948px]A.There was a greater number of students in the 2011 class than in the 2012 class.
[size=13.63636302948px]B.The percent of students who received ‘A’s in 2011 was less than in 2012.
[size=13.63636302948px][size=13.63636302948px]C.Five more students received ‘A’s in 2012 than in 2011.
D.The same number of students received ‘B’s in 2011 as in 2012.
E.The percent of students who received ‘B’s was greater in 2012 than in 2011.


答案给的是E。

E要invalidate结论的条件是2011的平均分要不低于B，并且2012得A的比例也没有明显变高。

同样C需要的条件是2012的学生人数没有变多，得B的比例没有降低。我理解有问题吗？

hongshaomj

显示不好，重写编辑一下

In a freshman biochemistry class at Newton University, the teacher assigns the class only ‘A’s, ‘B’s’ or ‘C’s. The average on the semester final for the class of 2012 was five points lower than that of the class of 2011. Therefore, the percent of students who received ‘C’s was greater in 2012 than in 2011.

Which of the following, if true, suggests the conclusion above is not necessarily valid?

A.There was a greater number of students in the 2011 class than in the 2012 class.
B.The percent of students who received ‘A’s in 2011 was less than in 2012.
C.Five more students received ‘A’s in 2012 than in 2011.
D.The same number of students received ‘B’s in 2011 as in 2012.
E.The percent of students who received ‘B’s was greater in 2012 than in 2011.