The sum of the digits of a two-digit number is 9 less than the number. Which of the following digits is at unit's place of the number ?

A 1

B 2

C 3

D data inadequate

Correct Answer: Option D

Let the ten's digit be x and unit's digit be y. Then, (10x + y) - (x + y) = 9 or x = 1.

From this data we cannot find y, the unit's digit. So the data is inadequate..