Which of the following actions involves voluntary muscle movement?
movement of food through the digestive tract
movement of blood through the blood vessels
beating of the heart
making of faces

Making of faces - this is the correct answer in this case because the movement of either food or blood or beating of the heart for that case don't involve voluntary muscle movement. On the other hand, when're making faces, we're deliberatly deciding whether we want to make a face and what face we want to make. 

They create genetic variations