Answer:
let's use c as circumference, d for diameter
lets say c increases, d is related to c, d will also increase
if c decreases, d will too.
Let's put in this way.
Pretend you're at a store,
The more you spend, the more points you get in their loyalty program
C is going to be the amount you spend
D will be how many points you get
Spend more, more points
spend less, less points
P.S
C is the independent variable, D is the dependent variable. (I think that's the right way, if not ignore the postscript aka P.S)