Determine how many strings of n lowercase letters from the English alphabet contain

(a) the letter a.

(b) the letters a and b.

(c) the letters a and b in consecutive positions with a preceding b, with all letters of the string distinct.

(d) the letters a and b, where a is somewhere to the left of b in the string, with all letters distinct.

2. Determine how many strings of n lowercase letters from the English alphabet contain

(a) the letter a.

(b) the letters a and b.

(c) the letters a and b in consecutive positions with a preceding b, with all letters of the string distinct.

(d) the letters a and b, where a is somewhere to the left of b in the string, with all letters distinct.