Random digits are integers selected from among {0,1,2,3,4,5,6,7,8,9} one at a time in such a way that at each stage in the selection process the integer chosen is just as likely to be one digit as any other. In simulation experiments it is often necessary to generate a series of random digits by using a random number generator. In generating such a serie, let X denote the number of trials needed to obtain the first zero. a) What is the functional form of the pmf? b) Find the P(X=3). c) Find P(X<=5). d) What is the mean of X? e) What is the Var(X)?