The components in the circuit shown below are ideal. If the op-amp is in positive feedback and the input voltage $V_{i}$ is a sine wave of amplitude $1$ $\text{V}$, the output voltage $V_{o}$ is
1. a non-inverted sine wave of $2$ $\text{V}$ amplitude.
2. an inverted sine wave of $1$ $\text{V}$ amplitude.
3. a square wave of $5$ $\text{V}$ amplitude.
4. a constant of either $+5$ $\text{V}$ or $-5$ $\text{V}$.