Collapse into two dimensions: imagine a rectangle formed by the diameter of the cylinder as the base and the height of the cylinder as its height.
Then, the distance from A to B will split this rectangle into two equal right triangles with
base = diameter = 6 and
height = 8 (the height of the cylinder).
You'll notice that this 6:8:x right triangle, where x is the hypotenuse, is our old friend the 3:4:5 triangle (in this case, 6:8:10), so x must equal 10.
The answer is D.
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