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The equivalent inductance measured between the terminals $1$ and $2$ for the circuit shown in the figure is

1. $\mathrm{L}_{1}+\mathrm{L}_{2}+\mathrm{M}$
2. $\mathrm{L}_{1}+\mathrm{L}_{2}-\mathrm{M}$
3. $\mathrm{L}_{1}+\mathrm{L}_{2}+2 \mathrm{M}$
4. $\mathrm{L}_{1}+\mathrm{L}_{2}-2 \mathrm{M}$