Please answer with clear explanation

Grog the gym owner bought four new machines at $ 20,000 each . Grog 's old machines , while a few years old , are still worth $ 4,000 per piece . He sells three of his old machines on Craigslist and gives away two of them to members of his gym for free . The net increase in Gross Domestic Product(GDP) of these transactions was

a ) $ 62,000 .

b ) $ 80,000 .

c ) $ 98,000 .

d ) $ 92,000 .