College graduates earn much more, on average, than high school graduates. Research done in the US finds that high school grads over an adult's working life earn an average of $1.2 million, associate's degree holders earn about $1.6 million, and bachelor's degree holders earn about $2.1 million. According to economists, this difference in earnings is mostly due to __________________
a) compensating differentials
b) having network connections
c) differences in human capital
d)luck