In cardiac cycle, the filling of ventricular blood mainly depends on: ()

A、thesqueezingeffectofaatrialcontraction;
B、the"suction"effectofventriculardiastolic;
C、intrathoracicnegativepressurepromotesvenousbloodcirculationtotheheart;
D、bloodreturndependingongravity;
E、thesqueezingeffectofeskeletalmusclepromotesvenousbloodcirculationtotheheart