question: Why is December 21 thought to be the birthday of basketball ?
answer: It was on this day that the first game of basketball was played .
turker_answer: December 21 is thought to be the birthday of basketball because it was on this day that the first game of basketball was played .
rule-based: December 21 is thought to be the birthday of basketball because of it was on this day that the first game of basketball was played .