On a Graph of Wage Rates Versus Risk of Injury,indifference

On a graph of wage rates versus risk of injury,indifference curves are convex because

A) risk of injury decreases workers' utility.
B) each additional dollar of pay increases utility more than the previous dollar.
C) utility is constant on indifference curves.
D) at low levels of risk, a worker is less willing to give up wages for increased safety.