Equivalent expressions SAT question

Choice B is correct. Applying the commutative property of multiplication, the expression $(m^4{q}^4{z}^{-1})(m{q}^5{z}^3)$ can be rewritten as $(m^{4}m)(q^4{q}^5)(z^{-1}{z}^3)$. For positive values of $x$, $(x^a)(x^b)=x^{a+b}$. Therefore, the expression $(m^{4}m)(q^4{q}^5)(z^{-1}{z}^3)$ can be rewritten as $(m^{4+1})(q^{4+5})(z^{-1+3})$, or $m^5{q}^9{z}^2$.

Choice A is incorrect and may result from multiplying, not adding, the exponents.

Choice C is incorrect and may result from conceptual or calculation errors. 

Choice D is incorrect and may result from conceptual or calculation errors.