Minecraft Redstone detecting players standing on a block in a radius!

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So, basically, I need to make it so that if a player is standing on a block of black wool in a radius of 50 (from the contraption), it will kill them instantly. how is this possible, if at all? I can't get it to work so that it will execute the 'kill @a' part for the people standing on wool. I know how to do it with a set of co-ordinates, but the area I have chosen is very large, and would be much faster and efficient to use [r=50]
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How do you detect if a player is standing on a specific block?
2. Type the CommandHow To Detect When A Player Is Standing On A Block In Minecraft || Insta Kill Or Launch Pad
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