A biology lab class wants to test whether pond snails reproduce more effectively in warm or cold water. The students place two small artificial ponds next to one another under a tree and add 12 snails to each pond. They place a heater in one and a chiller in the other. They count the number of egg masses at the end of each week for one month. What is the independent variable in their study?

the temperature of the pond
the number of snails in each pond
the amount of bird poop falling into the ponds
the number of egg masses each week