First off, nothing is impossible. It is possible to hack any website, even Google, if you have the right tools and knowledge. But to actually cheat or hack a site like Alexa it would take so much time and work that you could be putting into making your website more popular in a legit way. And besides, by the time you did succeed (which I doubt would ever even happen), Alexa would notice and in 20 minutes all your hard work is for nothing.
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