Why did giraffes develop long necks?

1) Describe an experiment to test this hypothesis. Be explicit about the methods you will use, the setting, the time that the experiment will last, and the variables you will measure.

2) Describe what types of results will illustrate your hypothesis or not. Make use of graphs to illustrate potential outcomes of the experiment.

I have to admit I'm a little unsure of what I should do, which is why I was asking for a little guidance. In a nutshell, I was thinking of saying that scarcity in food caused giraffes with long necks to demonstrate higher reproductive fitness. This is because longer necks allow them to reach longer branches where there would be less competition for food amongst other giraffes and other species. Is this good? Please critique.