Which of the following hypotheses should be used to investigate whether having more items in their cart is associated with an increased time to check out?
a) H0: β = 0; Ha: β < 0
b) H0: β = 0; Ha: β > 0
c) H0: β = 0; Ha: β ≠ 0
d) H0: b = 0; Ha: b ≠ 0