都应成为学习的管理者。在学习过程中应该（    ）

①学会自主学习

②提高学习效率

③科学安排时间

④套用他人好的学习方法和策略

    Singapore is not just a clean and tidy city in the world, it's also the greenest. There are (A) 三个原因why it is so.
Say "No" to waste
    Singaporeans do not waste a single drop of water. They recycle water and save( B ) almost all rainfall. The water can be used for industry （工业） and even drinking.
Many "green" buildings
    The Duke-NUS Graduate Medical School is one of the great examples of (C) pretty green design. It uses sustain-able design elements (可持续发展的设计元素) in its school building. ( D ) Singapore has many green buildings like this.
Get them while they're young
    The Green Singapore 2050 (GS2050) is a community platform (社区平台) for young people to express their great worries about environmental problems, and think of solutions for(E) them. Why 2050'？It's because these young people will be the ones to run the country in 2050, and hopefully they will solve the world's problems.

费老师在介绍我国古代某一朝代市民生活的讲座中，多次提及“瓦子”“勾栏”。由此推断，这一朝代应是（    ）