5. Given $\log _{8} 3=a, \log _{3} 5=b$, then the value of $\lg 5$ is
A. $\frac{3 a+b}{5}$
B. $a^{2}+b^{2}$
C. $\frac{3 a b}{1+3 a b}$
D. $\frac{1+3 a b}{a+b}$
5. C $\log _{2} 3=3 a, \log _{3} 5=b, \log _{2} 5=3 a b, \lg 5=\frac{\log _{2} 5}{1+\log _{2} 5}=\frac{3 a b}{1+3 a b}$

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5. C $\log _{2} 3=3 a, \log _{3} 5=b, \log _{2} 5=3 a b, \lg 5=\frac{\log _{2} 5}{1+\log _{2} 5}=\frac{3 a b}{1+3 a b}$