Which Example Is Most Clearly a Case of Density-Dependent Population

Multiple Choice

Which example is most clearly a case of density-dependent population regulation?

A) the summer drying of savanna grass for an insect that feeds on grass sap
B) a dangerous new flu strain that is transmitted among humans by sneezing
C) the first hard frost of fall for a population of annual morning glory vines
D) the occurrence of rainstorms for an opportunistic desert annual