5. (10 points) Fill in the appropriate digits in the vertical format to make the vertical format valid, then the sum of the first vertical format (which is the minuend of the second vertical format) is ( )
A. 1000
B. 1001
C. 1002
D. 1003
【Answer】Solution: According to the subtraction of a four-digit number by a three-digit number, the sum of the previous vertical expression is 1000 or 1001, and the subtrahend is 998 or 999,

Based on the two addends, the minimum is 900 and 101, with a sum of 1001, so it can only be 1001. Therefore, the answer is: $B$.