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In the differential amplifer of the figure, if the source resistance of the current source $I_{E E}$ is infinite, then the common-mode gain is

1. zero
2. infinite
3. indeterminate
4. $\frac{\mathrm{V}_{m 1}+\mathrm{V}_{m 2}}{2 \mathrm{V}_{\mathrm{T}}}$