Which of the following is correct regarding crystal systems?

• A

  Cubic

  $a \neq b = c$

  $\alpha = \beta \neq \gamma = 90^{\circ}$

  $Cu, KCl$

• B

  Monoclinic

  $a \neq b = c$

  $\alpha = \beta = \gamma = 90^{\circ}$

  $PbCrO_2, PbCrO_4$

• C

  Rhombohedral

  $a = b = c$

  $\alpha = \beta = \gamma \neq 90^{\circ}$

  $CaCO_3, HgS$

• D

  Triclinic

  $a = b = c$

  $\alpha \neq \beta \neq \gamma \neq 90^{\circ}$

  $K_2Cr_2O_7, CuSO_4 \cdot 5H_2O$