Which of the following contributions to the theory of the conservation of energy is an observation of a biological system? a. julius von mayer’s observation that vibration increases the temperature of water b. james prescott joule’s observation that the potential energy of a weight can be converted into heat energy c. hermann von helmholtz’s demonstrations that energy is converted but not created when animal muscles move d. baron kelvin’s hypothesis that the melting point of ice decreases as pressure decreases