$3.$ State whether each item below would be included or excluded from the measure of GDP and explain why. $(12$ points $3$ points for each part. $2$ points for the answer and $1$ point for the explanation$)$

a$.$ Income received, a credit card or check payment, by a mechanic for servicing your car.

b$.$ A person who works on their own car.

c$.$ The purchase of $100$ shares of corporate stock.

d$.$ A $$5$ billion increase in business inventories.