某植物种群AA基因型个体占30%，aa基因型个体占20%，该种群的A和a基因频率分别是（）。
A.30%、20%
B.45%、55%
C.50%、50%
D.55%、45%
参考答案：D
解析：种群中，AA基因型个体占30%，aa基因型个体占20%，Aa占1-(30%+20%)＝50%。A的基因频率为30%+50%／2＝55%。a的基因频率为1-A＝1-55%＝45%。
One Monday morning，news got around in the schoolyard about a fire over the weekend.Soon Mr.Johnson，my teacher，came to explain that the Art class would be ____ due to the fire.He also warned us not to approach the Art room because it could be ____. During the morning break，Kirn and I decided to take a ____.A first hand observation would make us heroes at lunch.At the appointed time，Kirn and I ____ the Art room，ignoring the consequence I would face ____ I got caught.We stood on tiptoes looking hard ___ the door window，trying in vain to see ____ was forbidden.Suddenly a firm __ on our shoulders stopped us. Turning around，I found a teacher staring angrily at us.In those seconds millions of ___ flashed through my mind.My father would beat me hard.____ than that，my mom might be so mad that she might not stop him.My spirits ____ even lower while realizing how ____ Mr.Johnson would be. When Kirn and I were led back to our classroom，I could found ____ to hide and wanted to melt into the ____.However，Mr.Johnson had a different ___ to teach. Slowly，he bent down to look me in the ____，and I was met with a look of concern rather than anger.He spoke softly as he explained why he was ____ in my decision to go where I may have got hurt.He cared more about my ____ than the fact that I’d broken a rule.My heart was ___ with regret，and I discovered that he still loved me even after I disappointed him.The rest of the day，he didn’t even mention it in class publicly，___ did he tell my parents.It was the first time I felt the power of being given an unexpected second chance.
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