Discussion

Answer: C

To find the unit digit of the sum, we look at the unit digits of the individual factorials. \(1! = 1\), \(2! = 2\), \(3! = 6\), \(4! = 24\) (unit digit 4). For any \(n \ge 5\), \(n!\) will have a unit digit of 0, as it contains factors of 2 and 5. Therefore, we only need to sum the unit digits of the first four factorials: \(1 + 2 + 6 + 4 = 13\). The unit digit of the sum is the unit digit of 13, which is 3.