The maximal active tension the muscle could develp at the optimal initial length, this is because

A、at this initial lenght, the most number of cross bridge could be involved in the filament sliding.;
B、at this initial length, the concentration of Ca2+ in the intracllular fluid reaches the highest.;
C、at this initial length, the speed of the filament sliding is fastest.;
D、at this initial length, the ATPase at the myosin head is most active.