根据下列句子及所给单词的首字母，在答题卷上按题号写出各单词的完全形式（每空限填一词）。
⑴My grandfather likes to water the f____  in the garden on weekends.
⑵The boy is too young. So don't leave him a____  at home.
⑶If I don't help my mom do some h____  on weekends, my mother will be very angry with me.
⑷Last night I didn't go to bed u____  the TV program was over.
⑸Last Sunday I w____  a letter to my father.
⑹I need to look smart for my job i____ .
⑺He f____  his test last week. He should study hard.
⑻He m____  his teacher in the street yesterday.
⑼When the little girl saw the big tiger, she felt very s____ .
⑽Last week, I p____  500 yuan for my new bike.

—Three days.

按一对夫妇生两个孩子计算，人口学家统计和预测，墨西哥等发展中国家的人口翻一番需20～40年，美国需40～80年，瑞典人口将会相对稳定，德国人口将会减少。预测人口的这种增减动态主要决定于(    )