What would decrease减退 the population of caterpillars

Diagram shows a food web for a farm农场.

Q) What would decrease减退 the population of caterpillars?
......D) II, IV

I. Plant more vegetables
II. Breed more chickens
III. Kill the sparrows
IV. Kill the snakes

A) I, III
B) I, IV
C) II, III
D) II, IV