Can I use Adsense as back-up ads?

Hello everyone,,
Some ad networks have options to show other ads if there are no ads available. Can I use Adsense the back-up ads for these networks? One good example is Chitika. If Chitika has no available ads, they can either fill the space with a solid color, collapse the space or display ads from other networks. Can I use Adsense as back-up? Is it against Adsense TOS??

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