Manuel and three other students are studying the effects of various factors on the growth of plants in a garden. Manuel is asked to investigate the effects of abiotic factors that involve the geosphere. The other three students are investigating effects that involve the biosphere, hydrosphere, and atmosphere. Which factor would be entirely Manuel's responsibility to investigate?

A. the volume of rainwater that soaks into the garden soil over one month
B. the sizes of rocks mixed into the garden soil
C. the species of insects, worms, and other small animals that live in the garden soil
D. the rate of decay of dead plant matter in the garden soil