At any point on the graph, the height of the line above the x-axis
shows the car's speed at that time.
Since the car is slowing down in the time between 10s and 30s,
the graph must be dropping lower and lower during that time.
Also, since you said that the car's speed decreases steadily,
the slope of the graph is constant, and the graph is a straight
line.
So, during that time, the graph is a straight line, sloping downward.
(Choice 'C'.)
