What is the sum of the described series? Use . Sn=(a1+an/2)

A company has agreed to make a yearly monetary gift to a charity. The first year it donates $2,000. Each year after, it promises to increase the donation by $500. How much total money will the company have given to the charity after 15 years?

A.
$82,500
B.
$77,000
C.
$9,000
D.
$5,500