What is the difference between tax rates and tax rate structures? How does this notion of 'structure' relate to the vert

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What is the difference between tax rates and tax rate
structures? How does this notion of 'structure' relate to the
vertical equity of a tax system? Select the INCORRECT response.
a.
A tax rate is the ratio of tax revenue to a tax base as set by
statute such as an income tax rate or a mil rate.
b.
A tax rate structure is the set of rates related to a particular
tax base.
c.
For the income tax, vertical equity can be categorized by
observing the tax rate structure (how rates respond to income).
d.
Taxpayers face many different tax structures but only one tax
rate.