The tetragonal crystal system has which of the following unit cell parameters?

• A
  $a = b = c$ and $\alpha = \beta = \gamma = 90^o$
• B
  $a = b \neq c$ and $\alpha = \beta = \gamma = 90^o$
• C
  $a \neq b \neq c$ and $\alpha = \beta = \gamma = 90^o$
• D
  $a = b \neq c$ and $\alpha = \beta = 90^o, \gamma = 120^o$