Read this passage from your science course. (The passage is from the lesson on speed.) Speed—also called average speed—is an idea you live with every day. Sometimes you want to know how fast something is moving or can move. You may be interested in auto racing and want to know how fast one of the cars can go. You may be a goalie on your soccer team—and you want to know how fast the ball can be coming at you. To calculate speed, you can use a simple scientific formula r = d/t. The r stands for rate of motion (speed), the d stands for distance, and the t stands for time. So speed is equal to distance divided by time.

Which answer best describes the structure of this passage?

A. a definition of speed, followed by several examples of speed in action

B. description of event related to speed, followed by a definition of average speed

C. everyday examples of questions about speed, followed by the rule for figuring out speed

D. rules for calculating speed in different settings, followed by examples