Mr. Draper asks his class to design a system of gears, strings, and pulleys that will allow him to pull back the classroom's curtains simply by turning a crank at his desk. The system must be built within the following constraints: It must be made only out of metal gears, metal pulleys, string, and one crank. It must be strong enough to move the curtains. It cannot be in the way of students or their desks. It must take no more than one day to build. The final design will use which type of energy to perform work on the curtains? A. elastic potential energy B. thermal energy C. gravitational potential energy D. kinetic energy