3. The following table shows information (number of contracts) of futures trading at each time point. All futures trading was done by four people A, B, C, and D. In the quantity in the trader part of the table, (+) means buy, (-) means sell, and the shaded part means there was no transaction. In the table, write the trading volume and open interest at each point in the blanks.

point of view t1 t2 t3 t4 A +3 -1 B +1 -3 -2 -1 trader -4 -1 +1 D +4 +2 +1 + trading volume open interest