The metallic radii of Group 13 are $\mathrm{Ga}$ 135 pm, $\mathrm{In}$ 167 pm and $\mathrm{Tl}$ 170 pm. The increase from $\mathrm{Ga}$ to $\mathrm{In}$ is 32 pm but from $\mathrm{In}$ to $\mathrm{Tl}$ only 3 pm. The most appropriate explanation for the small second increase is:

1. ATl has the same number of shells as In
2. Bthe inert pair effect shortens the $\mathrm{Tl}-\mathrm{Tl}$ metallic bond only
3. Cthe filled $4f$ (and $5d$) subshells inserted before Tl shield the nucleus poorly, so $Z_{\mathrm{eff}}$ is high and cancels most of the effect of the additional shell
4. DTl atoms lose the $6p$ electron and become cations in the metal