Which of the following geometric series converges?
∑n=1∞(1.1)n\sum_{n=1}^{\infty} (1.1)^n∑n=1∞(1.1)n
∑n=1∞(0.9)n\sum_{n=1}^{\infty} (0.9)^n∑n=1∞(0.9)n
∑n=1∞(−1)n\sum_{n=1}^{\infty} (-1)^n∑n=1∞(−1)n
∑n=1∞2n\sum_{n=1}^{\infty} 2^n∑n=1∞2n