如图，在直径为AB的半圆O上有一动点P从A点出发，按顺时针方向绕半圆匀速运动到B点，然后再以相同的速度沿着直径回到A点停止，线段OP的长度d与运动时间t之间的函数关系用图象描述大致是(    )

    I live in a small town near Xing'an in Guilin. You can't see it on the map of China, because it is too small. The air here is fresh(清新的). There are not many tall buildings in our town. The tallest building is in our school. There are four hundred students and twenty-five teachers in our school. In the front of the school, there is a playground. I often play basketball with my classmates on it. There is a little garden behind our school. And we can grow(种植) beautiful flowers and plant trees there. Next to the garden, there is an orange orchard(果园). You can hear birds singing everywhere. There is a river not far from our school. In summer, we usually go swimming with our teachers in it. We study Chinese, English, Maths and other subjects at school. The teachers are very nice. We love our school.