题干

一种果蝇的突变体在21℃的气温下,生存能力很差,但是,当气温上升到25.5℃时,突变体的生存能力大大提高了。这说明                                     (     )

A:突变是不定向的

B:突变是随机发生的

C:突变的有害或有利取决于环境条件

D:环境条件的变化对突变体都是有利的

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答案(点此获取答案解析)

C

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Dear Dad,
   Happy birthday to you !
   On this special day I'd like to talk to you1a special way.
   I don't think I am good at2you my thought face to face with you,so I'm writing to show my deep love for you.You're not a rich man or a famous person.But in my heart,you are one of the greatest3 in the world.I am4 you.
   You are5 interested in fame and wealth.You do6 things like paying phone bills on time,and working as a worker in your factory.The smile on your face shows you are pleased with the family.You take good care of my grandparents.You help me with my schoolwork and do some shopping with Mom on Sundays.
   In the past,I didn't care7 you were with me or not.Now I am sorry to say I8 you the respect before.But I am thankful for what you have done for me. I am quite lucky that I have9 a great father.And I feel happy that now I can let you10 how much I love you.You are successful as a son,a husband,a father,and a friend.
   Wish you happy forever!
                                                                                                                                               Yours,

                                                                                                                                               David