Which of the following is NOT a true statement about flywheels for energy storage? a. The value of k for a spherical shell is 2/3. b. Flywheels have been used for many years as a method of evening out fluctuations in the rotational speed of a shaft due to varying torque. c. The mechanical properties of the flywheel material are of fundamental importance for the design. d. The most advantageous geometry for a flywheel is a solid disk. e. The maximum amount of energy that can be stored by a flywheel depends upon the breaking strength of the flywheel material