单项选择题

A.剪力在梁的左断面时向下为正,在梁的右断面时向上为正
B.剪力在梁的左断面时向下为负,在梁的右断面时向上为负
C.剪力在梁的左断面时向下为正,在梁的右断面时向上为负
D.剪力在梁的左断面时向下为负,在梁的右断面时向上为正