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There are many benefits to timing your practice, including:
For 1, the distance from (2,4) to (5,10) is sqrt(3^2 + 6^2), so it is clear that the minimum diameter is greater than 6 (the smaller circle is when (2,4) and (5,10) are at opposite ends of the circumference (which is the boundary of the circle).
For B, the minimum radius is sqrt(13)/2 + 1.803, and it can be as large as we want.