Let A be the set of 4-digit numbers ${\mathrm{a}}_1{\mathrm{a}}_2{\mathrm{a}}_3{\mathrm{a}}_4$ where ${\mathrm{a}}_1<{\mathrm{a}}_2<{\mathrm{a}}_3<{\mathrm{a}}_4$ then n (A) is equal to

• A

  84

• B

  126

• C

  210

• D

  None of these