到两定点A（0，0），B（3，4）距离之和为5的点的轨迹方程是           (        )
3x–4y="0," 且x＞0                     4x–3y="0," 且0≤y≤4   
4y–3x=0,且0≤x≤3                   3y–4x=0,且y＞0