A Building Owner Charges Net Rent of $20 in the First

A building owner charges net rent of $20 in the first year, $21 in the second year, and $22 in the third year, but is providing six months of free rent in the first year as a concession. Using a 10 percent discount rate, what is the effective rent over the three years?

A) $17.28
B) $20.00
C) $20.94
D) $21.00
E) $21.73