Which statement about the slope of a horizontal line is true?

Which statement about the slope of a horizontal line is true?

• A. It is undefined
• B. It is positive
• C. It is negative
• D. It is zero

Answer: (D)

Slope measures how y changes as x changes. For a horizontal line, there is no change in y as x changes, so the rise is zero while the run is a nonzero amount. That makes the slope rise over run equal to 0 divided by a nonzero number, which is 0. This is why a horizontal line has slope zero. The other possibilities describe different cases: a vertical line has an undefined slope because the run is zero; a line that rises as you move right has a positive slope, and a line that falls has a negative slope. So the statement that the slope is zero is the correct one.

Slope measures how y changes as x changes. For a horizontal line, there is no change in y as x changes, so the rise is zero while the run is a nonzero amount. That makes the slope rise over run equal to 0 divided by a nonzero number, which is 0. This is why a horizontal line has slope zero. The other possibilities describe different cases: a vertical line has an undefined slope because the run is zero; a line that rises as you move right has a positive slope, and a line that falls has a negative slope. So the statement that the slope is zero is the correct one.