At a Taxable Income of $40,000 Mari's Income Tax Is

At a taxable income of $40,000 Mari's income tax is $7,400.When her taxable income rises to $45,000 her income tax is $8,400.Based on this information,what is Mari's marginal tax rate?

A) 18.7 percent
B) 39 percent
C) 10 percent
D) 30 percent
E) 20 percent