The answer is wave B has a lower frequency. Suppose these waves represent the sound of a siren on a passing ambulance. Wave B (lower frequency) represents the sound of the siren after it has passed you. The ambulance moving towards the observer produces a higher than normal frequency, and the ambulance moving away produces a lower than normal frequency.
It is harder to push a box with groceries in it because the mass is weighing the box down. Which makes it harder to push
