Final answer:
Speed is the distance travelled per unit time, and both 'b distance/time' and 'd d/t' are correct answers for its definition. The rate of change of velocity is known as acceleration.
Step-by-step explanation:
Speed is defined as the distance travelled per unit time. It is a scalar quantity, which means it only has magnitude and no direction. Thus, the correct answer to 'Speed is _________________________' would be both 'b distance/time' and 'd d/t', as they express the same relationship. When dealing with the rate of change of velocity, this is known as acceleration. Acceleration can result in a change in the magnitude or direction of velocity, or both. You can calculate the average acceleration by taking the change in velocity and dividing it by the elapsed time. This concept is crucial in understanding motion in physics.