题目

某DNA分子中胸腺嘧啶的数量为m，占该DNA分子总碱基数的比例为q，若此DNA分子连续复制n次，需游离的鸟嘌呤脱氧核苷酸数为( )      A．(2n－1)m  B．m(1/2q－1)      C．(2n－1)m/2nq  D．(2n－1)m(1－2q)/2q 答案：D D     Five years ago，David Smith wore an expensive suit to work every day．“I was a clothes addict.” he jokes. “I used to carry a fresh suit to work with me so I could change if my clothes got wrinkled．”Today David wears casual clothes—khaki pants and sports shirt—to the office．He hardly ever wears a necktie．“I’m working harder than ever，”David says，“and I need to feel comfortable．”     More and more companies are allowing their office workers to wear casual clothes to work in the United States．The change from formal to casual office wear has been gradual．In the early 1990s，many companies allowed their employees to wear casual clothes on Friday(but only on Friday)．This became known as“dress—down Friday”0f“casual Friday”．“What started out as an extra one—day—a—week benefit for employees has really become an everyday thing．”said business consultant Maisly Jones．       Why have so many companies started allowing their employees to wear casual clothes? One reason is that it’s easier for a company to attract new employees if it has a casual dress code．“A lot of young people don’t want to dress up for work，”says the owner of a software company，“so it’s hard to hire people if you have a conservative dress code．”Another reason is that people seem happier and more productive when they are wearing comfortable clothes．In a study conducted by Levi Strauss and Company，85 percent of employers said that they believe that casual dress improves employee morale．Only 4 percent of employers said that casual dress has a negative impact on productivity．Supporters of casual office wear also argue that a casual dress code helps them save money．“Suits are expensive，if you have to wear one every day，”one person said．“For the same a—mount of money，you can buy a lot more casual clothes．” 68、David Smith refers to himself as having been“a clothes addict’’because        .      A、he often wore khaki pants and a sports shirt    B、he couldn’t stand a clean appearance    C、he wanted his clothes to look neat a11 the time    D、he didn’t want to spend much money on clothes 69、David Smith wears casual clothes now，because       . A、they make him feel at ease when working B、he cannot afford to buy expensive clothes C、he looks handsome in casual clothes D、he no longer works for any company 70、According to this passage，which of the following statements is FALSE?   A、Many employees don’t like a conservative dress code． B、Comfortable clothes make employees more productive． C、A casual clothes code is welcomed by young employees． D、All the employers in the U．S．are for casual office wear． 71、According to this passage, which of the following statements is TRUE? A、Company workers started to dress down about twenty years ago． B、Dress—down has become a11 everyday phenomenon since the early 1 990s． C、“Dress—down Friday”Was first given as a favor from employers． D、Many workers want to wear casual clothes to impress people． 72、In this passage，the following advantages of casual office wear are mentioned EXCEPT       . A、saving employees money B、making employees more attractive      C、improving employees motivation D、making employees happier