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Both transistors $\mathrm{T} 1$ and $\mathrm{T} 2$ show in the figure, have a threshold voltage of $1$ Volts. The device parameters $\mathrm{K}_1$ and $\mathrm{K}_2$ of $\mathrm{T} 1$ and $\mathrm{T} 2$ are, respectively, $36 \; \mu \mathrm{A/V}^{2}$ and $9 \; \mu \mathrm{A} / \mathrm{V}^{2}$. The output voltage $V_{0}$ is

1. $1 \mathrm{~V}$
2. $2 \mathrm{~V}$
3. $3 \mathrm{~V}$
4. $4 \mathrm{~V}$