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ACCorDing to thE proFEssor, why DoEs phoBos risE so oFtEnA．it is ClosE to thE surFACE oF mArs.
B．it is muCh smAllEr thAn othEr moons.
C．it movEs vEry slowly ArounD mArs.
D．it is not rEAlly A moon At All.