In a classic experiment carried out by the biologist hopkins in the 20th century, 20 young rats were divided into two groups. one group was fed on a diet of pure carbohydrate, lipid and protein; the other was fed the same diet, but with some added milk. the rats were weighed every day. after 18 days the diets of the groups were swapped over.
why did hopkins use 2 sets of rats?
suggest some important control variables that hopkins kept constant during his investigation.

there are mainly three domains in biological classification namely; archaea, bacteria, and eukarya.

archaea includes all the prokaryotic cells which are characterized by their lipid membranes. they have branched hydrocarbon which is attached to glycerol through an ether linkage.

bacteria includes all the prokaryotic cells, the cell membrane of which are made up of the unbranched fatty acid chain. in addition, they are attached to glycerol through an ester bond.

eukarya includes all the eukaryotic cells, that is, the cells contain a nucleus and other membrane-bound cell organelles.

they can be autotrophic as well as heterotrophic, unicellular as well as multicellular

they include kingdoms like protista, plantae, animalia, fungi et cetera.

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explanation: