how does the one who asks choose the best answer i do not find any provision?

Don't confuse rating and voting. Rating is available at level 2 and just indicates good answers, not necessarily the best. The best answer gets one point for each thumbs up. Level 2 is at 250 points.

✔ For Y̲o̲u̲r̲ Questions:

You must wait 1 hour before you can pick a best answer. Go to "My Activity/My Q&A" where you will find a list of all your questions. Select the question and there will be a "vote as best answer" button. You receive 3 points for picking one. You will have 4 days to pick one or it will go to a vote. You can also force the vote to start immediately on the edit (pencil) menu.

✔ For S̲o̲m̲e̲o̲n̲e̲ E̲l̲s̲e̲'s̲ Questions (and your own after 4 days):

You will have to wait 4 days if the asker did not pick it himself. There will be a "vote as best answer" button. You receive 1 point for voting. You must vote before the vote closes after 2 more days. You are allowed to vote for your own answer. You just can't answer your own question. Look for questions you can vote on by clicking on the "in voting" tab.

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