CEMC Pascal, 2005

Combinatorics Difficulty 2.0 Junior Find the answer Canada

A whole number is called decreasing if each digit of the number is less than the digit to its left. For example, 8540 is a decreasing four-digit number. How many decreasing numbers are there between 100 and 500?

(A) $11$ (B) $10$ (C) $9$ (D) $8$ (E) $7$

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