It is thought that approximately 90% of plant species went extinct during the Carboniferous Period (~320-290 mya). Which of the following graphs represent the changes that likely occurred in carbon dioxide and oxygen levels? (The solid black line depicts CO2 levels, the dashed line represents O2 levels) The left side of the x-axis represents the time of the extinction event. a. graph Lb. graph Hc. graph Kd. graph M