Which one of the following unit cell parameters represents a tetragonal crystal system?
- $\mathrm{a}=\mathrm{b}=\mathrm{c} ; \quad \alpha=\beta=\gamma \neq 90^{\circ}$
- $\mathrm{a} \neq \mathrm{b} \neq \mathrm{c} ; \quad \alpha=\beta=\gamma=90^{\circ}$
- $\mathrm{a}=\mathrm{b} \neq \mathrm{c} ; \alpha=\beta=90^{\circ}, \gamma=120^{\circ}$
- $\mathrm{a}=\mathrm{b} \neq \mathrm{c} ; \quad \alpha=\beta=\gamma=90^{\circ}$
