What was the control in the experiment?

What was the control in the experiment?

Select one:
a. The varying amounts of salt added to the soil
b. Plants grown in typical soil
c. The length of time the plants were grown
d. 50 plants per trial

Plants grown in typical soil are the control.

The experiment is a process which includes the testing of effects several substances on a known event. Here in this experiment, the effect of salt concentration in soil on the plants are to be studied.

Control is a set of results that are kept for better verification of the results of the experiment. Here the control is kept in ideal or typical conditions and the effect on the plants are tallied with that of plants from experiment.

So the control here should be those plants which are grown in normal soil. Or soil of the environment where the plants are indigenous.