Two continuous random variables X and Y have a joint probability density function (PDF), f subscript X Y end subscript left parenthesis x comma y right parenthesis equals 2 e to the power of negative x minus y end exponent comma space 0 less than x less than y less than infinity. The conditional PDF of Y given X=x, i.e., f subscript Y vertical line x end subscript left parenthesis y right parenthesis, is 

A. fraction numerator 2 e to the power of negative x minus y end exponent over denominator integral subscript x superscript infinity 2 e to the power of negative x minus y end exponent d y end fraction equals e to the power of x minus y end exponent comma space y greater than 0 

B. fraction numerator 2 e to the power of negative x minus y end exponent over denominator integral subscript 0 superscript infinity 2 e to the power of negative x minus y end exponent d y end fraction equals e to the power of negative y end exponent comma space y greater than 0 

C. fraction numerator 2 e to the power of negative x minus y end exponent over denominator integral subscript 0 superscript infinity 2 e to the power of negative x minus y end exponent d y end fraction equals e to the power of negative y end exponent comma space y greater than x greater than 0 

D. fraction numerator 2 e to the power of negative x minus y end exponent over denominator integral subscript x superscript infinity 2 e to the power of negative x minus y end exponent d y end fraction equals e to the power of x minus y end exponent comma space y greater than x greater than 0