研究历史离不开文字资料。我国有文字可考的历史开始于（   ）

《中共中央关于全面推进依法治国若干重大问题的决定》提出，将每年十二月四日定为国家宪法日，建立宪法宣誓制度，凡经人大及其常委会选举或者决定任命的国家工作人员正式就职时公开向宪法宣誓。这体现了（   ）。

①中共中央行使最高决定权，决定党和国家的重大事务

②宪法是党的主张和人民意志相统一的体现，必须崇尚宪法权威

③***坚持依宪执政，推动建设中国特色社会主义法治国家

④***领导立法、带头守法，坚持依照宪法行政

根据短文理解，选择正确答案。

    Winter-swimming has become popular in Beijing. Three years ago, few people would go swimming in the city waters. But now there is a Winter-swimming Enthusiasts' Club(冬泳爱好者协会)and it has more than 2,000 members. The oldest is 84 years of age and the youngest is only 7. The members are from all ways of life__They may be workers, peasants, soldiers, teachers, students...

    Though it is now the coldest part of the season and the water temperature in the city's lakes is around 0°C, many winter-swimmers still swim in the icy waters, even when it is snowing. They enjoy themselves in the lake, while the people by the side of the lake are wearing heavy clothes.

    Why are so many people interested in winter-swimming? Because winter-swimming can be good for one's health.

    Bei Sha is a good example. He is 69 years old, and he once suffered from heart trouble for 26 years. After ten years of winter-swimming he is now in good health.

    Scientists are now studying the effects of winter-swimming on health.

摆长为l的单摆做简谐运动，若从某时刻开始计时(即取t＝0)，当振动至t＝时，摆球恰具有负向最大速度，则单摆的振动图象是下图中的(　　)