$2-2$ One of your firm's suppliers discounts prices for

A larger quantities. The first $1000$ parts are $$13$ each. The next $2000$ are $$12$ each. All parts in excess of $3000$ cost $$11$ each. What are the average cost and marginal cost per part for the following quantities?

$($a$)500$

$($b$)1500$

$($c$)2500$

$($d$)3500$