The circuit shown in the figure has an ideal opamp. The oscillation frequency and the condition to sustain the oscillations, respectively, are

- $\frac{1}{CR}$ and $R_1=R_2 \\ $
- $\frac{1}{CR}$ and $R_1=4R_2 \\ $
- $\frac{1}{2CR}$ and $R_1=R_2 \\ $
- $\frac{1}{2CR}$ and $R_1=4R_2 $