What could have done to change something vs what must have done to change something

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Looking on the internet deeply has found these results:

what could have done to change something is the most popular phrase on the web. 

what must have done to change something

12,020,000,000 results on the web

what could have done to change something

12,290,000,000 results on the web

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Some examples and use cases from the internet:

Some examples and use cases from the internet:

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