Aresearcher is interested in the effects of nitrate and phosphate on plant growth and sets up an experiment in which groups of five plants are given 1, 2, and 3 grams of nitrate and 1, 2, and 3 grams of phosphate in all combinations over a period of one month. the researcher measures plant height and weight at the end of the experiment. what is missing in this experimental design?
a. a control
b. an independent variable
c. a dependent variable
d. a constant

The correct answer will be option-A

Explanation:

A scientific experiment is designed in a way that it tests the predicted hypothesis and so contains the three different types of variables namely: independent (can be modified), dependent (to be studied) and the control (kept constant and unchanged).

In the given question, since the scientists studied the effect of nitrate and phosphate in which he provided the plant samples with nitrate and phosphate but he missed the control sample which should lack the variable to be tested that is nitrate and phosphate. This control variable helps the scientist to study the relationship between other samples.

Thus, option-A is the correct answer.

the question isn't complete here so i can't right an answer '_'