Answer:
Joint variation.
Step-by-step explanation:
A joint variation is where a variable depends on two other variables, that is, it can change because of two other variables, and it's a direct relation, if both variables increase the dependent variable also increase, and vice versa.
In this case, we see that c depends on x and y, if any of those two variables increase, c will also increase.
Therefore, this is joint variation.
