lisney

the problem with this argument is that it takes the observation that "the number of Kravonians enrolled in college has been growing steadily", and takes that observation to mean that the number of graduates is going to increase in proportion.

choice (b) attacks this connection -- if students are taking longer to graduate from college, then the increased enrollment doesn't necessarily mean that there will be any more graduates! it's just a symptom of the fact that students are staying in school for longer.

for instance, if students take 4 years to graduate from college and college graduates 1000 students per year, then (not counting dropouts) that college's enrollment will be approximately 4000 students.
if students begin to take five years to graduate from college, then that same college will begin to have an enrollment of 5000 students -- even if it graduates the same number (1000) of students per year.
也就是说由于在学校呆的时间长了，使得在校人数增加了，但是每年毕业的人数没有增加