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Im afraid mining is no longer possible. you can become a witness tho.

If mining is not possible, then how Steemit generate Steems for their rewarding? Is mining banned for the internet users and only Steemit consortium is doing the mining?

mining is replaced by witnesses, a witness signs a block just like a miner would do, but to be a witness you have to be voted in . so the witnesses (miners) are delegated by the majority of steemians voting for them.

So, how to become a witness? Do you have any idea?

Yes. I am a witness. I can point you in the right direction.
I will get back to you with some information.
Feel free to contact me in chat :-)