A research project studied the physical properties of wood materials constructed by bonding together small flakes of wood. One of the questions of interest is the effect of the flake size and tree species on the tension modulus of elasticity in the direction perpendicular to the alignment of the flakes, in pounds per square inch (psi). This project considered two flake sizes, 0.15 inches by 2 inches and 0.25 inches by 2 inches, and three tree species: aspen, birch, and maple. For each combination of the flake size and tree species, three samples of wood material were constructed, and measure the tension modulus of elasticity. If we restate the question of interest in terms of the relationship between the response variable and explanatory variable,

(a) Regarding the response variable, [ Select ] ["there are 6 response variables, which are the combinations of two flake sizes and three tree species.", "there are two response variables, which are the flake size and tree species.", "there is one response variable, which is the tension modulus of elasticity."] .

(b) Regarding the explanatory variable, [ Select ] ["there are 6 explanatory variables, which are the combinations of two flake sizes and three tree species.", "there is one explanatory variable, which is the tension modulus of elasticity.", "there are two explanatory variables, which are the flake size and tree species."] .