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In the CMOS inverter circuit shown, if the transconductance parameters of the NMOS and PMOS transistors are $k_{n}=k_{p}=\mu_{n} C_{o x} \frac{W_{n}}{L_{n}}=\mu_{p} C_{o x} \frac{W_{p}}{L_{p}}=40 \; \mu \mathrm{A} / \mathrm{V}^{2}$ and their threshold voltages are $V_{T H n}=\left|V_{T H p}\right|=1 \mathrm{~V}$, the current $I$ is

1. $0 \mathrm{~A}$
2. $25 \; \mu \mathrm{A}$
3. $45 \; \mu \mathrm{A}$
4. $90 \; \mu \mathrm{A}$