you must download API 21 and install the Minimum SDK per your requirement. This way you can run your application from the early Froyo (if you keep the Minimum SDK as 8) phones on Lolipop.
So, when you start with Android Development, you will find configuration options in AndroidManifest.xml like MinimumSDK version (the minimum version of android you want to support), TargetSDK (maximum version of OS u want to support) version and CompileSDK (usually equal to the target version of the SDk) .
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