Determine the value of the constant c such that the following functions are valid probability functions:
A. p(x) = cx/20, x = 1,2,3,4,5,6 and zero elswhere.
B. p(x) = c(X^2 + 1), x = 1,2,3,4,5, and zero elsewhere.
C. p(x) = c(x -1 ), x = 2,3,4,5,6 and zero elsewhere.