Which of the following is true for a cubic crystal system?

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