How many variables should be manipulated in a controlled experiment?

Which type of organism is missing from the food web seen in the illustration? primary consumers decomposers primary producers secondary consumers

This is a typical grassland food web. it is also a small picture of an important cycle on earth: the carbon cycle. describe how the carbon gets into this food web. a) bacteria and fungi, the decomposers, recycle carbon from dead organisms. b) carbon is found in the grass and is passed from one level to the next in this food web. eliminate c) all living things give off carbon dioxide as a by-product of respiration and it is released into the atmosphere. d) plants use carbon dioxide as a reactant in photosynthesis, to make usable chemical energy in the form of a sugar.

A. describe the scientific method, using pasteur's swan-necked flask experiment as an example.