Select the one substance that is not a rock.

 - Conglomerate is a solid. It is formed in nature.
 - An antler is made by a living thing. It is not a pure substance.
 - Slate is not a pure substance. It is formed in nature.
Compare the properties of each substance to the properties of rocks. Select the substance whose properties do not match those of rocks.
An antler is made by a living thing. But rocks are not made by living things.
So, an antler is not a rock.
Conglomerate is a rock.
Slate is a rock.