A 1-kg object is moving with a velocity of 6 m/s to the right. It collides and sticks to a 2-kg object moving with a velocity of 3 m/s in the same direction. How much kinetic energy was lost in the collision? 
选项为:A:$$1.5J$$  B:$$2J$$  C:$$2.5J$$  D:$$3J$$  请回答其中一个选项.