Dana designs an experiment to test the speed of sound waves as they travel through different materials. Her research question is, "How do different materials affect the speed of sound waves as they travel? What would be the test (independent) variable in Dana's experiment?
O amount of time it takes the waves to go through the material
O type of material
O the tool used to measure the time
O the object used to make the sound

You can look it up online and some images should pop up 

In reversible reaction chemistry equilibrium is attained when the forward and reverse reactions are taking place at the same rate, thus, the concentrations of reactants and products remain the same. a reversible reaction is a chemical reaction where the reactants form products that, in turn, react together to give the reactants back. in irreversible reactions on the other hand, the reactants react to form the products, which cannot revert back into reactants.