In a Market,to Find the Total Amount Supplied at a Particular

In a market,to find the total amount supplied at a particular price,

A) we must add up all of the amounts that firms are willing and able to supply at that price.
B) we must take the average of the amounts that firms are willing and able to supply at that price.
C) we must add all of the costs that firms incur to supply the product at that price.
D) all determinants of demand must be taken into account.