S is a set of arithmetic expressions as strings recursively defined as follows. Base case: Every variable from the set { a, b, c, d, e, f } is in S. Recursive rules: If x E S and y E S, then: 1. ( x + y) ES 2. X . YES Indicate which expressions are in S. (a + b + c) (a . d) + (c . b) (a + d) (e+ f) a . b