Which conjunction or disjunction is equivalent to the open sentence |a – 9| ≤ 4. a. a – 9 ≥ 4 and a – 9 ≤ –4 b. a – 9 ≤ 4 or a – 9 ≥ –4 c. a – 9 ≤ 4 and a – 9 ≤ –4 d. a – 9 ≤ 4 and a – 9 ≥ –4