Create a semi-circle (the method doesn't matter but I happened to use a straight line to slice a circle).
Using one of the semi-circles:
Set line width to 'none' and make the fill a light grey.
Clone it, make the clone black then enlarge clone to be 110%.
Put clone at back then select both objects and align top edges.
Why do you get a very thin line showing through the light grey semi-circle from the object beneath?
I've attached a .xar and .jpeg to illustrate.
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