For his science project, Austin wants to determine if the pH of standing rainwater changes over time. He collects a jar of rainwater and places it indoors in a spot that does not receive a lot of sunlight to limit evaporation. Using a pH testing kit, he measures the pH of the water once a day for 20 days.

What is the independent, or manipulated, variable in Austin’s experiment?

a. the number of days the rainwater is allowed to sit
b.the number of times the rainwater is observed
c. the change in the amount of rainwater due to evaporation
d.the change in pH of the rainwater over time