Google Adsense Scam?

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I was running google adsense on my sites and earned about $175 from last 3 months, but when I asked them for payout, they informed that my account is closed due to invalid clicks. As far as I know, I have not clicked on my site any time. How, they can say that I have got invalid clicks. Does google really pay or Adsense is also a big scam?