In the diagram, which letter represents the activation energy? A B C D A graph in three sections. Horizontal axis is labeled Reaction progress and has no scale. Vertical axis is labeled Energy and has no scale. Section 1 of the graph is a horizontal line segment that begins about halfway up the length of the vertical axis. Section 2 is a concave down parabolic arc that begins where segment 1 ends. It increases to a height of about three quarters the length of the vertical axis, then decreases to a height of about one tenth the length of the vertical axis. Section 3 of the graph is a horizontal line segment that begins where section 2 ends, at a height of about one tenth the length of the vertical axis. The graph has four labels: A, B, C, and D. Label C represents the height of section 1 of the graph. Label D represents the height of section 3 of the graph. Label A represents the difference in the height in the graph between the peak of the parabola in section 2 and the height of the graph in section 1. Label B represents the difference in the height of the graph in section 1 and the height of the graph in section 3.