The unit digit of \((3547)^{153} \times (251)^{72}\) is:
Answer: C
We need the unit digit of \(7^{153} \times 1^{72}\). The unit digit of \(1^{72}\) is 1. For \(7^{153}\), the cycle is (7,9,3,1), length 4. We find the remainder of \(153 \div 4\), which is 1. So the unit digit is the 1st in the cycle, which is 7. The unit digit of the product is the unit digit of \(7 \times 1 = 7\).