Why would a person choose to tie a helium-filled balloon to a child's wrist instead of simply having the child hold on to it? a. helium is less dense than air, so it will float above air if the child lets go of the balloon. b. helium is more dense than air, so it will sink below air if the child lets go of the balloon. c. helium is less dense than air, so it will sink below air if the child lets go of the balloon. d. helium is more dense than air, so it will float above air if the child lets go of the balloon.