GAUSS7 2000 Problem 6

Past Contests GAUSS7 GAUSS (GR. 7) - 2000
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In the addition shown, a digit, either the same or different, can be placed in each of the two boxes. What is the sum of the two missing digits?
gauss7 2000 6

Answer Choices
A. 9
B. 11
C. 13
D. 3
E. 7