It is the last choice because their similar bone structures suggest they once had a common ancestor, which is why they are all mammals.
One of the most general forms of discomfort or pain one feels at the time of strenous work out is a burning sensation in the muscles or lungs, which goes away after some time, that is, after stopping the activity. This is a result of an accumulation of lactic acid.
Lactic acid is a by-product of the procedure the body goes through when it requires to generate energy more briskly that it does usually, like when one exercises.
The muscles functioning generally produce energy aerobically, that is, by using oxygen, however, when one push himself or herself at the time of workout and enough oxygen is not accessible, then these muscles start producing energy anaerobically, resulting in production of lactic acid as a by-product and ultimately causing burning sensation.
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Viruses reproduce within the living cells of the host and use the cellular machinery to synthesize their own genome and other components. To gain entry into cells, they have developed a variety of mechanisms to introduce their genes and proteins into host cells.
The answer is <span>d. homologous structures.
Homologous structures are </span><span>similar structures shared by different species. For example, the anatomy of joints in bones of birds and dinosaurs, support the idea that these two groups have a common ancestor. But since these two groups are different enough to be separated into different classes, it can be concluded that they evolved from the common ancestor.</span>