Humans can cause an ecosystem to collapse by
a. collecting and chemically altering industrial waste to detoxify it.
b. reducing habitat fradmentation.
c. hunting a keystone species to extiction.
d. building shelter belts next to farm fields.

C.Hunting a keystone species to extinction.

Explanation:

The ecosystem is an interaction of living beings with their non-living physical environment. The living organisms interact among themselves so as to sustain their lives.

The human beings are responsible for the collapse of ecosystem by their interventions and ill-legal activities. The keystone species is the one on which the other species of the ecosystem are dependent upon for their survival. If the keystone species are hunted at an level of extinction this is likely to affect other species of the ecosystem and will negativity affect their diversity.  

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.