A biology student is studying ways to grow crops in soils that are high in salts and other dissolved solids. To determine whether corn plants can tolerate salty soil conditions, she grows 50 corn plants in typical garden soil and 50 corn plants in the same garden soil plus 0.01 kilograms of salt. After 6 weeks of growth, the corn plants grown in salty soil were the same mass and height as the corn plants grown in non-salty garden soil. The student concluded that salt had no effect on the growth of corn.

What comment would you make to the student regarding her experimental design and conclusion?
A) The experimental design included both a control and an experimental group. The conclusion is valid.
B) The student should have used more plants in each sample group; the larger the sample size, the more valid the results.
C) The conclusion is not valid. Increasingly high amounts of salt should have been added to more corn plots for comparison.
D) Although the conclusion is valid, next time the student might want to grow corn in plots with different water and temperature conditions as well.

The experimental design to test a hypothesis must include all the parameters to be tested. The experimental design should include two types of samples: experimental groups in which the independent variable can be manipulated and the control group in which the variable to be studied must be absent.

In the given experiment the student tried to study the tolerance of the corn plants to salinity. So the amount of salt must be provided in different concentrations in experimental samples. The different amount of salt will help the student to compare the tolerance of corn plants in different concentration and will lessen the errors.

Thus, Option-C is the correct answer.