WordPress Archive using only one category

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Hello,

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I want to use the archive widget on my site but it uses every category e.g Dog, Cat, Bears etc. Except I only want it to show one i.e Cat.

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This should be simple to fix but apparently it isn’t because I can’t seem to figure out how to stop it displaying posts from all categories.

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Any suggestions?

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Would prefer to avoid plugins if possible.

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Thanks.

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