【题目】某质点参与 _(1)=5cos (pi t-dfrac (pi )(2))cm 及-|||-_(2)=5cos (pi t+dfrac (pi )(2))cm 两个同方向、同频率的简谐振动,-|||-则合振动的振幅为 ()-|||-A.5cm-|||-B.10cm-|||-C.15cm-|||-D.20cm