A scientist who wants to study the effects of fertilizer on plant growth sets up an experiment. Plant A gets no fertilizer, Plant B gets 5mg of fertilizer each day, and Plant C gets 10mg of fertilizer each day. What is the independent variable?

Independent variable: Days

Reason: if you were to create a graph, days would be along the x axis and the results of the appliance of the fertilizer would be along the y axis. It is generally known that the units along the x axis are labeled as the independent variable because they don't rely on the results to display said effects on each day.