Indicate which of the following statements concerning the following tax rate structures is/are correct.

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When Income Total Tax
Equals Equals
Structure #1 10,000 600
100,000 5,000

Structure #2 15,000 900
75,000 4,500

Structure #3 13,000 975
86,000 6,600
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I. Tax Structure #1 is proportional.
II. Tax Structure #1 is regressive.
III. Tax Structure #2 is progressive.
IV. Tax Structure #3 is progressive.
a. Only statement I is correct.
b. Only statement III is correct.
c. Only statements I and II are correct.
d. Only statements II and IV are correct.