Choice A is the correct answer. Calculate using half-lives.

1. Half-lives: $\frac{2112}{704} = 3$ half-lives.
2. Remaining: $(\frac{1}{2})^3 = \frac{1}{8}$.
3. Result: $\frac{1}{8}$ of original amount remains.

💡 Strategic Tip: 3 half-lives = $(\frac{1}{2})^3 = \frac{1}{8}$.

Choice B is incorrect because this is after 2 half-lives. Choice C is incorrect because this is after 1 half-life. Choice D is incorrect because this would be after 4 half-lives.