There are 20 balls in an urn, of which x are red balls and rest are black balls. If y red balls and z black balls are added to the urn. If a ball is selected at random, what is the probability that the selected ball is a black ball?
A. \(\dfrac{x+y}{20+y+z}\)
B. \(\dfrac{20-x+y}{20+y+z}\)
C. \(\dfrac{20-(x+z)}{20+y+z}\)
D. \(\dfrac{x+z}{20+y+z}\)
E. \(\dfrac{20-x+z}{20+y+z}\)
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