There are several different models that represent compounds. One type of model is shown.

3 C's connected in a line by 2 black lines. The first and third C's each have 3 H's connected to them by a single black line each. The middle C has 2 H's connected to it by a single black line each.
What type of model is shown?

a structural formula
a chemical formula
a space-filling model
a ball-and-stick model