For the following scenario, at the end of which year should you replace the defender? Defender; EUAC

Question:

For the following scenario, at the end of which year should you replace the defender?
Defender;
EUAC 1: 9,102, EUAC 2: 6,414, EUAC 3: 6,822, EUAC 4: 9,991.
TC 1: 9,102, TC 2: 6,012, TC 3: 6,391, TC 4: 10,682.
Challenger
EUAC 1: 8,162, EUAC 2: 8,869, EUAC 3: 9,843, EUAC 4: 9,002.