In many semiconductors, a biexciton can be formed as a bound state of two excitons, analogous to a hydrogen molecule formed from two hydrogen atoms. For the Γ₆ conduction band and Γ₇ and Γ₈ valence bands discussed in the above example in T_d symmetry, find the possible symmetries of the biexciton states by computing all the possible outer products of two exciton states.