For three rational numbers a/b, c/d, and e/f , we have: 1. a/b - (c/d - e/f) = a/b - c/d + e/f 2. a/b - (c/d - e/f) = a/b - c/d - e/f 3. a/b + (c/d + e/f) ≠ (a/b + c/d) + e/f 4. a/b + (c/d + e/f) = (a/b + c/d) + a/b + e/f