6. Insert a one-digit number (including 0) into a two-digit number, and it becomes a three-digit number, for example, inserting 6 into 72 results in 762. Some two-digit numbers, when a one-digit number is inserted, become a three-digit number that is 9 times the original two-digit number. How many such two-digit numbers are there?
(A) 1.
(B) 4
(C) 10
(D) More than 10

Please retain the original text's line breaks and format, and output the translation result directly.
6. B(Because "three-digit number" is 9 times "two-digit number", the sum of "three-digit number" and "two-digit number" should be 10 times "two-digit number". Therefore, the unit digit of this "two-digit number" can only be 0 and 5. Since the inserted digit should be less than the unit digit of the "two-digit number", the unit digit of the "two-digit number" can only be 5, and the inserted unit digit can only be 0, 1, 2, 3, 4. Combining with the characteristic of being divisible by 9, only 0, 1, 2, 3 are valid, and the corresponding "two-digit numbers" are 45, 35, 25, 15. )