If the common ratio $r < 1$, the sum of the first $n$ terms of a geometric series is —

• a$\dfrac{a(1 - r^{n})}{1 - r}$
• b$\dfrac{a(r^{n} - 1)}{r - 1}$
• c$a(1 - r^{n})$
• d$\dfrac{a + r^{n}}{1 - r}$