When borax is heated with \( \mathrm{CoO} \) on a platinum loop, blue coloured bead formed is largely due to:
\( \mathrm{B_2O_3} \)
\( \mathrm{Co(BO_2)_2} \)
\( \mathrm{CoB_4O_7} \)
\( \mathrm{Co[B_4O_5(OH)_4]} \)