Tidal heating occurs when gravitational forces cause a body like a moon to deform, which creates heat. This happens whenever something is deformed due to internal friction. However, for heat to be sustained, the moon must continually deform, like an accordion being played. What kind of orbit is required in order to induce sustained tidal heating? 

a. A circular orbit

b. An elliptical orbit 

c. A parabolic orbit 

d. A hyperbolic orbit.