Graph A: A horizontal line goes from (1, 0.5) to (2, 0.5). Another horizontal line goes form (2, 0.2) to (7, (0, 2). Graph B: A curve starts at (0, 0), curves up to (1, 1), and then curves down to (2, 0). Which graph represents a density curve, and why?

graph A only, because the curve is above the horizontal axis, and the area under the curve from 2 to 7 is 1
graph B only, because the curve is above the horizontal axis, and the area under the curve is equal to 1.57
both graph A and graph B, because both curves are above the horizontal axis, and both areas are positive
neither graph A nor graph B, because, even though both curves are above the horizontal axis, neither graph has an area of 1