3.) a scientist is testing the efficiency of various ph levels by measuring average growth rate. she runs 7 trials. what is a control variable of this experiment? a.) plant growth b.) ph of soil c.) average plant growth d.) type of soil

Option D, Type of soil

Explanation:

A control variable is the one which remains unaltered throughout the experiment so that relationship between other variables could be established accurately.  

Here, the impact of pH is to be studied on the growth rate of plant.  However, we can see that the plant type, pH level and growth rate are continuously changing. Only the soil type is consistent throughout.  

Hence, option D is correct

explanation:

plz give me the characteristics.

well, i believe that the answer is 2. : 3