As Part of a Research Project, a Graduate Student Creates

As part of a research project, a graduate student creates an artificial environment in a sealed container. She sterilizes the soil and adds plants, snails, shrews, and a snake. After adding a small artificial pond, she seals the container. During the first several weeks, the plants are thriving and the animals are doing well. However, piles of dropping are beginning to accumulate, and the entire floor of the container becomes covered with a layer of plant debris several inches thick. One probable cause for the problems is that the graduate student didnʹt

A) put in enough plants.
B) add any decomposers.
C) include any carnivores.
D) include any autotrophs.
E) include any herbivores.