What is the amount of oxygen in the atmosphere?

Actually depends on the Textbook you use.

Its usually quantified in percent and its usually 21%.

The correct option is A. All valid research usually have two types of variables, dependent variables and independent variables. These variables allow a researcher to determine the exact relationship between the variables. In the question given here, if the scientist want to find out the reason for length difference in the marsh grass, he has to conduct an experiment using dependent and independent variables.