Recall that a trit is a symbol in {0,1, 2}. We are interested in trit strings of length 9, i.e., elements of {0, 1, 2}9. How many such strings of 9 trits are there that

(a) contain at least one 1? Explain and show all your calculations. (b) contain exactly one 1 and exactly one 2? Explain and show all your calculations. (c) contain exactly one 1 or exactly one 2? If you want to refer to the correct solutions of (a) and/or (b), you can use the variables xa and xb, respectively. Explain and show all your calculations.