what is the simplified version of the following expression

Answer:
D. 5^8/9^18
Step-by-step explanation:
The applicable rule of exponents is ...
(a^b)^c = a^(bc)
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[tex]\left(\dfrac{5^4}{9^9}\right)^2=\dfrac{5^{4\cdot 2}}{9^{9\cdot 2}}=\dfrac{5^8}{9^{18}}[/tex]