Curare is a substance that, when ingested, blocks a receptor for acetylcholine in muscles, causing paralysis. Curare is thus an example of an substance that is an for acetylcholine.

sound moves fastest in solids because the molecules are tightly packed and it allows sound to vibrate

the rocks are classified by the geological process they are formed by. the three main groups of rocks are igneous, metamorphic, and sedimentary. minerals are not they are classified under the chemical classes they are in. these groups include silicates, sulfates and carbonates.