There are two windmills near Brittany's town. Brittany wonders which one converts more wind energy into mechanical energy. What is the best way for Brittany to find out which windmill generates more mechanical energy?
A. Measure the height of each windmill.
B. Stand next to each windmill to feel where the wind is strongest. C. Find out which windmill was built more recently.
D. Count how many times each windmill spins in one minute.