为什么人们吃的食物要多样化？

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    Read the following advertisement and answer the questions below according to the table.

    SECRETARY TV

    Major network looking for secretary/gal/guy. Friday to work on various temporary assignments. Work 1，2，3，4 or 5 days a week. Mr. Newcomb 889-0030

    SECRETARY QUEENS

    Good skills, General insurance agency. Richmond Hill area.

    849-8760

    SECRETARY. P/T.

    Steno, type, hrs, Mon.-Fri., 10-1 and/or Mon. Wed. & Fri. 2:30-7:00.

    Tues.2:30-5:00 Midtown office 355-5743 after 7 p.m.

    SECRETARY

    Small midtown law firm. Excel skills, no legal exp req'd. 755-2040

    SECRETARY.

    Fast-moving ad agency needs bright, energetic, good typist for various responsibilities. 684-2500

    （P/T.=part time; Steno=stenographer速记员；exp=experienced；req'd=required）

     All students need to have good study habits. When you have good study habits, you will learn things quickly and also remember them easily. Do you like to study in the living room? This is not a good place, because it is usually too noisy. You need to study in a quiet place, like your bedroom A quiet place will help you only to think about one thing. When you study, do not think about other things at the same time. Only think about your homework. If you do so, you will do your homework more quickly, and you will make fewer mistakes.
    Good study habits are very important. If you do not have them, try to learn them. If your studyhabitsare already good. Try to make them better.

角α的终边在第一象限，则 $\frac{\sin \frac{\alpha }{2}}{\left|\sin \frac{\alpha }{2}\right|}$ + $\frac{\cos \frac{\alpha }{2}}{\left|\cos \frac{\alpha }{2}\right|}$ 的取值集合为（   ）

如图各容器中盛有海水，铁在其中被腐蚀时由快到慢的顺序是（   ）