A cataclysm is a violent, sudden event that causes great change and/or harm.
A catatonic person is in a state of suspended action; therefore, they are rigid, immobile, and unresponsive.
A catalyst is an agent that enacts change, such as speeding up a chemical reaction or causing an event to occur.
A catastrophe is a widespread terrible disaster that is often accompanied by many deaths.
A category is a class or group of similar things, such as fruit, nuts, or nouns.
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