The sum of the first $n$ terms of the series $5 + 55 + 555 + \ldots$ is —

• a$\dfrac{5}{81}\left(10^{n+1} - 9n - 10\right)$
• b$\dfrac{5}{9}\left(10^{n+1} - 1\right)$
• c$5n$
• d$\dfrac{5}{81}\left(10^{n} - 9n\right)$