On a distance vs. time graph, the line of an object at rest is a a. curved line with a slope of zero b. flat line with a slope of zero c. jagged line with a negative slope d. curved line with a positive slope

If an object is at rest, then its position ... or distance from something ... doesn't change as time goes on.  

Its graph is a horizontal line.  

(The slope of a horizontal line is zero.)  

(choice-b)

heats surface water up