What can cause a canker sore?

A stress or minor injury to the inside of the mouth is thought to be the cause of simple canker sores. Certain foods —including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes, strawberries) — can trigger a canker sore or make the problem worse.

Stress,injury, not cleaning where canker sore has formed before, and certain foods

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Ibelieve the answer is b,hope my answer has you : )