You can identify two variables from the statement: between latitude above the equator and average temperature, these are the two variables they looked at "latitude" and "average temperature".
The response variable is whatever we are interested in measuring.In this case. it's the average temperature. And the explanatory variable is what "explains" the value of the response of variable. So the explanatory variable is latitude.
An easy to identify these is to think of them like being dependent/independent variables, so the altitude is fixed and is your dependent variable, example explanatory variable,and the average temperature is dependent on latitude, so is the dependent variable example response variable.
