Answer:
A. Type of soil
Explanation:
A control experiment is the one that is used to checkmate the real experiment.
In this case, the fourth jar that contains the mixture of all the types of the soil is considered as the control experiment.
In a nutshell, it should be generally understood here that the type of soil serves as the control for the experiment.
Tropical climate is a region of the earth surrounding the equator. When the earth is exposed to light from the sun it is right over the equator and up 23 degrees of latitude Tropic of Cancer in the northern hemisphere and Tropic of Capricorn in the southern hemisphere