According to various aspects, exemplary embodiments are disclosed of apparatus and methods for retaining tennis balls hit into tennis nets, to inhibit or prevent the tennis balls from bouncing off or rolling away from a tennis net back onto the main playing surface of a tennis court. The agent may be used to minimize organ rejection, transplantation-mediated transmission of viral infection, and triggering of apoptosis in transplanted tissues. Furthermore, the continuously variable valve lift system is provided at a position level with or below a position at which the cam shaft is installed, thus reducing the overall height of the cylinder head, thereby reducing the volume of the engine.