It's because there's tar on it.
Because tar is a fat-soluble material, it does not dissolve in water but in fat-soluble substances such as oil.
If you buy a small amount of kerosene at the gas station and rub it with it, it will be erased.
If you search on the Internet for tar remover, there's something they sell.
You can erase it with that.
If you want to make it cheaper, you can use kerosene as your superiors say.
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