请根据上面的图片上的信息，创建讲解视频---**Problem Description:** 爷爷每天晚饭后从家出发去散步，中途在阅报栏读一会儿报后便回家，爷爷散步的时间与离家距离之间的关系如图所示。 **Chart Description:** * **Type:** Line chart. * **Main Elements:** * **Coordinate Axes:** X-axis labeled "时间 (分)" (Time (minutes)), Y-axis labeled "距离 (米)" (Distance (meters)). * **Scale:** X-axis has marked points at 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50. Y-axis has a marked point at 900. * **Data Series:** A line graph representing the relationship between time and distance from home. * Starts at the origin (0, 0). * Increases linearly from (0, 0) to (20, 900). * Remains constant at a distance of 900 meters from time 20 minutes to time 30 minutes (horizontal line). * Decreases linearly from (30, 900) to (45, 0). * **Annotations:** Dashed lines extend from the point at time=20, distance=900 to both axes, and from the point at time=30, distance=900 to both axes. **Questions:** (1) 图中反映了哪两个变量之间的关系？ (2) 爷爷出发后什么时候开始读报？读了多久？ (3) 爷爷每天散步多长时间？ (4) 爷爷散步时最远离家多少米？ (5) 计算爷爷回家前的15分钟内的平均速度？