Android_SDK4.2_ (API17) _Eclipse3.8 Development Environment has encountered a problem src folder red cross

Android eclipse

Android_SDK4.2_ (API17) _Eclipse3.8 red cross development environments encountered a problem src folder
I hope to have a big God answers, thank you.

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Without any log, did not describe any errors, help you really only God. . . .
Reply:
it is not associated with the project sdk
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build path
A look inside the package is not there with imported red cross, are generally not copy copy engineering reasons jar package
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Thank summer I somehow re-build