$A$ person wants to climb a $n$-step staircase using one step or two steps at a time. Let $C_n$ denote the number of ways of climbing the $n$-step staircase. Then $C_{18} + C_{19}$ equals

  • A
    $C_{20}$
  • B
    $C_{21}$
  • C
    greater than $C_{21}$
  • D
    less than $C_{20}$

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