What is the difference between a flat line
in a distance/time graph, and a
speed/time graph?

In a distance/time graph, a flat line means no movement is occurring. In a speed/time graph, it means there is movement (if the value for speed is above 0) but no acceleration or deceleration is taking place. The speed is constant.

the five characteristics of adolescence are biological growth and development, and undefined status, increased decision making, increased pressure and the search of self.