$25.$ Supply$-$side economic studies indicate that

a$.$ Marginal income tax rates of about $10\%$ start to reduce incentive to work.

b$.$ Marginal income tax rates of about $25\%$ start to reduce incentive to work.

c$.$ Marginal income tax rates of about $40\%$ start to reduce incentive to work.

d$.$ Marginal income tax rates of about $75\%$ start to reduce incentive to work.

Marginal income tax rates of about $90\%$ start to reduce incentive to work