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Sum of the digits of the product of a and b is 1 so a*b=10^n ( n is integer)
In addition, a and b are two positive integers that are not divisible by 10
=> a=2^n and b=5^n
The remainder when 5^n diveded by 10 is always 5
The remainder when 2^n divided by 10 could be 2;4;6;8
So the remainder when |a-b| is divided by 10 could be 3;1;9;7
Answer is B