let s be the set of all strings of 0's, 1's, and 2's that have length 4 and for which the sum of the characters in the string is less than or equal to 2. define a relation r on s as follows: the sum of the characters of string s equals the sum of the characters of string t. a. how many distinct elements are in set s? answer with an integer. b. how many distinct equivalence classes exist for relation r? answer with an integer.