I would take a small example to answer this.

3-9.

3-9 have 3 multiples of 3

9-3 / 3 = 2

So multiples inclusive of numbers will be 1 value greater than range by the number.

This is valid for any range. Hence the answer

This is an evenly spaced set so it is an arithmetic series also.
Now the nth term in the arithmetic series is given by:
an = a1 + (n-1)d
where n is the total no of terms in the series and d is the difference between the two consecutive terms.
We have an=729, a1=102 and d=3 as they are the multiples of 3.
So plugging in the values we get
729 = 102 + (n-1)3
After simplifying we get the A=B+1
So A is the answer.

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