Answer: C. outside temperature and heating bill
Step-by-step explanation:
A negative trend is when one variable goes up and the other variable goes down (or vice versa).
As the temperature goes down, the heating bill is likely to go up. This is of course assuming the person(s) use the heat to stay warm rather than just solely rely on something like coats/blankets/etc.
As the temperature goes back up, there's little to no need for heating. So the heating bill will likely go down.
As the name implies, negative trend graphs have negative slopes for the regression lines.