During the borax bead test with \( \mathrm{CuSO_4} \), a blue green colour of the bead was observed in oxidising flame due to the formation of:
\( \mathrm{CuO} \)
\( \mathrm{Cu} \)
\( \mathrm{Cu(BO_2)_2} \)
\( \mathrm{Cu_3B_2} \)