Which of the following dimensions represents a monoclinic crystal system?

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