Here's why:

* Cellular respiration is the process where cells break down glucose (a sugar) in the presence of oxygen to release energy in the form of ATP.

* Glucose is the fuel source for cellular respiration, meaning its presence and amount directly influence the process.

* Oxygen is another crucial component, but it acts as a reactant rather than an independent variable.

* The energy output (ATP) is the dependent variable – it changes depending on the amount of glucose available.

Therefore, the amount of glucose available is the independent variable that directly affects the rate of cellular respiration.