There are already 1,200 gallons of water in a swimming pool. If water is filling the pool at a rate of 20 gallons per minute, which expression indicates the amount of water, in gallons, in the swimming pool after m minutes? A. 1,200 + 20 × m B. 1,200 ÷ 20 × m C. 1,200 - 20 × m D. (1,200 + 20) ÷ m