A future contract with $200 delivery price is sold for $185 yesterday, and today its price changes to $188. The change in the future's price will require the buyer of this future contract to pay $3 difference in price to the seller of the future contract. This is an example of _____.

a. obligation in the future's contract

b. long position in the future's contract

c. exercise price of the future's contract

d. marketing to market in the future's contract