You can target any singular or multiple kw at once, but just make sure you have a really good content and good page authority to rank for multiple kw. Nice content will ensure the users to stay longer in the website and it'll tell google that you have a good content and google should shoot you to google page 1. Logically, after panda n penguin updates, don't over optimizing the on page by mentioning the same kw over and over again to rank for that kw. Be natural

Check it out first with Google Keyword Tool. Sometimes the plural gets searched more and at other times it is the singular. Go with the most searches

Use any keywords you can as long as they are within your niche. dont just target the single keyword ass others infront and behind it i.e ipad would become - buy ipad, ipad on ebay, ipad on amazon and so on.

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