The answer is not B because a new substance is not created, they just produced sparks and gas. The answer is not C because the liquid is still THE liquid even if it starts to boil. The answer is not D because that is a physical change called dissolving. So the answer would have to be A; blue liquid is mixed into a yellow liquid, and the mixture turns green. The two substances are being mixed together and created a new substance. Also a sign of chemical reaction is color change.