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JIM CARREY pulled another box office hit out of his hat, as his animated 'Horton Hears a Who!' topped the box office with this year's biggest opening so far: $45.1 million, according to preliminary studio estimates.

Carrey, in his second DR. SEUSS film (he also starred in 'How the Grinch Stole Christmas'), plays Horton the elephant, who hears voices coming from a speck of dust. Horton soon discovers, to his amazement, that a planet containing a village called "Who-ville" is located in the infinitesimal grain of dust and it's up to him to protect the people who live there. STEVE CARELL voices the Mayor of Who-ville, while DANE COOK, SETH ROGEN, CAROL BURNETT, JAIME PRESSLY, ISLA FISHER, JONAH HILL ('Superbad'), WILL ARNETT ("Arrested Development") and AMY POEHLER ("Saturday Night Live") also provide voices.

'10,000 B.C.' dropped to second place with $16.4 million. The ROLAND EMMERICH film follows a mammoth hunter as he goes on a quest that could make or break his tribe. STEVEN STRAIT and CAMILLA BELLE co-star.