Statement I: many two-digit numbers have digit sum 9 (18, 27, 36, 45, ...), not unique. Statement II: two-digit multiples of 9 are 18, 27, 36, 45, 54, 63, 72, 81, 90, 99; only 45 has tens digit 4, so the number is uniquely 45. Statement II alone is sufficient. Correct option: Statement II alone is sufficient, but Statement I alone is not sufficient.