某物体的运动规律为<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/af3d4aa255b44b6e996f5613de12cb3b.png?x-oss-process=image/sharpen,200"/>，式中k为大于零的常数。当t =0时，初速度为<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/cb7711d7ab78495ab98dca6c2374a5c2.png?x-oss-process=image/sharpen,200"/>,则速度<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/970d385e86bc42e9a3249872f47840db.png?x-oss-process=image/sharpen,200"/>与时间t 的函数关系是（）