One line has a rate of change of 4. A second line crosses it at a right angle. What is the rate of change of the second line?

1. Lines that cross at a right angle have rates of change that are negative reciprocals of one another. 2. Find the reciprocal first by dividing 1 by 4, which gives 0.25. 3. Then change the sign, which gives minus 0.25. 4. Check the pair by multiplying: 4 times minus 0.25 is minus 1, as it should be. 5. Option C keeps the reciprocal but forgets the sign change, so those two lines would never meet at a right angle. 6. Option B changes the sign but never takes the reciprocal. 7. Option D repeats the first rate, which would give a parallel line instead. _Source: OpenStax College Algebra (CC BY 4.0), Ch 4 "Linear Functions", section 4.1 Linear Functions_