Two cups with capacities of 9 liters and 3 liters, respectively, have different diameters at the top and bottom and are unmarked. If water can be taken arbitrarily, how many operations are required at least to pour out 6 liters of water? (Filling the cup is counted as one operation, pouring the water out is counted as another operation.) 
选项为:A:$$1$$  B:$$2$$  C:$$3$$  D:$$4$$  请回答其中一个选项.