When it comes to pop-music tricks, there is at least one that is just about infallible: make your chorus nonsense and highly melodic — see: sha la-la-la, Boh poh boh boh, naaaaa naaaa na na na na na, waka-waka, boom boom pow, etc. etc. — and watch everyone helplessly sing along. After the middling effort that was “Hold It Against Me,” Britney Spears’s next single, “Till the World Ends” (which comes from a bit of a shady provenance, so may not be in its final version), takes no chances. The song consists mainly of the extremely shameless, extremely irresistible chorus, “Oaah oaah oh-oh-oh-oh.” It’s the Venga Boys by way of world music, with a backing track of claps: Resistance is futile. (Plus, we all finally get a break from tortured double entendres.) Ke$ha co-wrote this song, and you can hear how it would work for her, with trippier, bleep-bloopier production values. But at this particular juncture, she needed a hit less badly than Spears, who, if she knows how to say thank-you properly, should be sending Key Dollar Sign Ha just as much teeth-brushing jack as a girl could ever ask for.