Which of the following is most clearly a case of density-dependent population regulation?
• the summer drying of savanna grass for aninsect that feeds on grass sap
• a dangerous new flu strain that istransmitted among humans by sneezing
• the first hardfrost of fall for a population of annual morning gloryvines
• the growth of shadetrees over a population of sun-loving shrubs in an abandonedfield
• the occurrence of rainstorms for anopportunistic desert annual