人口政策是一个国家根据本国人口发展状况而采取的相应政策措施，人口政策会随着本国人口发展的实际情况作适当的调整。刚刚解放的新中国实行鼓励生育的政策，那时一个家庭四五个孩子很正常，1982 年，计划生育确定为基本国策，提倡一对夫妇只生育一个孩子；2013 年 11 月启动实施一方是独生子女的夫妇可生育两个孩子的 （单独二孩）政策；2016 年 1 月 1 日起全面实施一对夫妇可生育两个孩子政策。读图“我国1949-2014 年出生人口统计图” 有不少学者曾预言 2010 年尤其 2013 年以后会出现第四次婴儿潮，理由可能是（    ）

①人口惯性             ②单独二孩政策实施       ③经济快速发展       ④医疗条件极大改善

若向量$\overrightarrow{a}$与$\overrightarrow{b}$不共线,$\overrightarrow{a}·\overrightarrow{b}\ne 0$,且$\overrightarrow{c}=\overrightarrow{a}-\left(\frac{\overrightarrow{a}·\overrightarrow{a}}{\overrightarrow{a}·\overrightarrow{b}}\right)\overrightarrow{b}$,则向量$\overrightarrow{a}$与$\overrightarrow{c}$的夹角为(    )

阅读下面材料，在空白处填入适当的内容（1个单词）或括号内单词的正确形式。

    This story took place in 1903. Adam, an American businessman, was carried out to sea by a strong wind. A ship ____ (spot) and rescued him, ____  was how he got to London by accident. He at once went to the American embassy ____ (ask) for help, but was refused. Two rich gentlemen invited him home because of a bet. They asked Adam a few questions, and Adam responded ____ a polite manner. They gave him a letter and asked him not to open it until two pm. Adam left wondering ____ would happen to him.