Explanation

Powers of 3 cycle in the units digit: 3, 9, 7, 1. Since 98 = 4·24 + 2, 3⁹⁸ ends in 9; since 89 = 4·22 + 1, 3⁸⁹ ends in 3. The units digit of the difference is 9 − 3 = 6 (no borrow needed at the units place). The answer is (B).