Suppose a futures contract on a stock index begins trading today. At 10 a.m., trader A goes long 5 contracts with trader B, who shorts 5 contracts, at a price of $200. At 2 p.m., trader A shorts 2 contracts with trader C, who goes long 2 contracts, at a price of $210. The settlement price of the day, set by the clearing house, is $205.

a. What is the volume of trades for the day?

b. What is the open interest at the end of the day?

c. What are the mark-to-market gains (losses) for each trader after settlement? Assume a multiplier (i.e., unit size) of 10.