Scientists believe that the earth's early atmosphere lacked gas because photosynthetic organisms did not yet exist. a hydrogen b nitrogen c oxygen d sulfur

Cells tend to have a relatively small and uniform size. why aren’t cells larger?

The gene that causes sickle-cell disease is present in a higher percentage of residents of sub-saharan africa than among those of african descent living in the united states. even though this gene causes sickle-cell disease, it also provides some protection from malaria, a serious disease that is widespread in sub-saharan africa but absent in the united states. discuss an evolutionary process that could account for the different percentages of the sickle-cell gene among residents of the two regions.

Astudent conducts an experiment to determine how the amount of water given to a plant affects it growth what is the dependent variable for this experiment