Simplify this expression :
[a⁻⁷/³ b⁴/³ / b⁻⁵/³]⁻³
All variables are greater than zero
a. a⁷ / b⁹
b. a⁹ / b⁷
c. b⁹ / a⁷
d. b⁷ / a⁹