I need to find the confounding variable

A.
Lemon trees receiving the most water produced the most lemons.

B.
An investigation found that more bushels of potatoes were produced when the soil was fertilized.

C.

Students measured the temperature of the water at different depths in Lake Skywalker and
found that the temperature varied.

D.

The amount of pollution produced by cars was measured for cars using gasoline containing
different amounts of lead.

E.

Four groups of rats are first massed and then fed identical diets except for the amount of vitamin
A they receive. Each group gets a different amount. After 3 weeks on the diet, the rats' masses are
measured again to see if there has been a decrease.