I tried making a rough sketch of what the graph might look like but I don't think I'm doing it correctly.
M(t) is positive because it measures time, so it could not be negative. Because the winning times are declining M'(t) is negative. It's the rate of change of winning times. If M"(t) is negative then the graph of M(t) is concave down, which contradicts the statement that times are declining at decreasing rate. I would put my money on M'(t) only if I am reading this correctly.