固体障碍物越远，回声返回所用时间就越长。
The further off this solid obstruction, the longer time will elapse for the return of the echo.
回声返回所耗的时间越长了。
固体障碍物越远。回声返回所用时间就越长。
The further off this solid obstruction, the longer time will elapse for the return of the echo.
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