The number of ways in which 20 letters  $x_1,x_2,\dots x_{10}$; $y_1,y_2,\dots ,y_{10}$ be arranged in a line so that suffixes of the letters x and also those of y are respectively in ascending order of magnitude is 

• A

  126

• B

  64

• C

  $2^{20}$

• D

  184756