问题：
[单选题]． 19.已知动点A沿其轨迹的运动方程为s=b+ct，式中b、c为常量，则()。
A、点A的轨迹必为直线
B、点A的轨迹必为曲线
C、点A必作匀速运动
D、点A的加速度必等于零
参考答案：
C
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