Which of the following unit cell parameters represents the tetragonal crystal system?

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