单选题 ______ crystal system has three axes (a₁, a₂ and a₃) in the x-y plane 120° apart and a fourth axis (z) at 90o to the x-y plane. The intercepts along the three axes in the horizontal plane are equal in length, but the fourth intercept, labeled c, is of a different length.
   A) Cubic
   B) Hexagonal
   C) Monoclinic
   D) Tetragonal