Can't use answers from the lesson because... well.. I don't have the lesson.
But here's what I do know:
As Kenneth Boulding once said:
"Anyone who believes in indefinite growth in anything physical, on a physically finite planet, is either mad or an economist."
This basically means that you can't have infinite economic growth in a finite environment, such as the earth.
This is also the argument for the switch to sustainable/renewable energy, which might be an example you could use. Sustainable energy is just that - energy that we can sustain for a theoretically infinite period of time. However, fossil fuels and oil are not sustainable because there is a finite amount of them on earth. We will run out.