Critical Reasoning

High school students who feel that they are not succeeding in high school often drop out before graduating and go to work. Last year, however the city's high school dropout rate was significantly lower than the previous year's rate. This is encouraging evidence that the program instituted two years ago to improve the morale of high school students has begun to take effect to reduce dropouts.

Which one of the following, if true about the last year, most seriously weakens the argument?

(A) There was a recession that caused a high level of unemployment in the city.
(B) The morale of students who dropped out of high school had been low even before they reached high school.
(C) As in the preceeding year, more high school students remained in school than dropped out.
(D) High schools in the city established placement offices to assist their graduates in obtaining employment.
(E) The antidropout program was primarily aimed at improving students' morale in those high schools with the highest dropout rates.

The answer given is A, and although this choice seems to be on the edge, is certainly the best choice.
However my question is not regarding the answer choice A. But an explantion given in the CR bible regarding choice B.
"At best, the answer choice is irrelevant. At worst, this answer confirms that some of the high school students had low morale, and in that sense, the answer strengthens the argument."

Premise:
High school students feeling that they are not succeeding(demoralized) - going to work.
Premise:
Last year, dropout rate plumetted compared to previous years
Conclusion:
The morale improvement drive has started to show good results, dropouts have decreased.

choice B says that morale of the few students who dropped out was already quite low:
its possible that many students who had a low morale might have dropped out even before reaching high shcool, which would explain the low dropout.
eg: out of 100 say 70 highschool students dropped out in the past,
This year 20 of 50 highschool students dropped out. ( 50 had dropped out even before reacing high school)

Why and how is the bible telling that this choice is irrelevant or strengthening the argument??