Lahars are a volcanic hazard that can occur both during an eruption and while a volcano is dormant. Which event below must happen for a lahar to form while a volcano is dormant?
A. the dormant volcano reaches isostasy
B. very heavy rainfall over the dormant volcano
C. the slope of the dormant volcano's flank exceeds its angle of repose
D. the opening of a fissure in the flank of the dormant volcano