The sum of the first $n$ terms of the series $7 + 77 + 777 + \ldots$ is —

• a$\dfrac{7}{81}\left(10^{n+1} - 9n - 10\right)$
• b$\dfrac{7}{9}\left(10^{n} - 1\right)$
• c$7n$
• d$\dfrac{7}{9}\left(10^{n+1} - 9n\right)$