Fluctuations in Graduate School Enrollment Correlate Positively with Fluctuations in Unemployment

Fluctuations in graduate school enrollment correlate positively with fluctuations in unemployment. What is the most likely reason why?

A) When jobs are difficult to find, the opportunity cost of going to grad school is high.
B) When jobs are easy to find, there are strong incentives to go to grad school.
C) The opportunity cost of going to graduate school is low when jobs are hard to find.
D) Even if you have a job, the salary will be low that going to graduate school is worth the effort.