Robert has measured the engine speed and horsepower of four different sports cars, listed the data in a table, and graphed the results on a scatterplot. He noticed the points fall closely on a line. Which of the following best describes the relationship between engine speed and horsepower based on Robert's observation?

A) There is no relationship between engine speed and horsepower.
B) Engine speed and horsepower are inversely proportional.
C) Engine speed and horsepower are directly proportional.
D) Engine speed and horsepower have a quadratic relationship.