Determine whether the given relation is reflexive, symmetric, transitive, or none of these. (select all that apply.) let a 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 a as follows: for every s, t is in a, s r t ⇔ the sum of the characters of s equals the sum of the characters of t.