External stimulus. The rain is something out of your control while you're waiting. Rain is not something you are doing nor is it a response to you waiting for the bus.
A grow and divide uncontrollably.
Answer: The scientist will perform another investigation in order to find out the role of certain genes in disease resistance.
Explanation-
As per the information given in the question, a mice having mutation in ear depicts same pattern of resistance when compared to another mice having normal ears. This indicates that the genes responsible for resistance are not related to genes that result in ear formation.
Therefore, to find out the effect of diet on disease resistance in rats, the scientist will perform another investigation in order to find out the role of certain genes in disease resistance.
