How many different four-digit numbers can be made using the digits 1, 2, 3, 4, 5, 6 if no digit can be used more than once? a. Subscript 4 Baseline C Subscript 2 Baseline = 6 b. Subscript 6 Baseline C Subscript 4 Baseline = 15 c. Subscript 6 Baseline P Subscript 4 Baseline = 360 d. Subscript 6 Baseline P Subscript 4 Baseline = 12