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    Welcome to Class 4, Grade 7. This is our classroom. There are twenty boys and twenty -four girls in the classroom. The teacher's desk is in the front of the classroom. There is a big blackboard on the front wall and a small blackboard on the back wall. There are two maps on the wall. One is a map of China and the other is a map of the world．There are some photos on the wall. There are some flowers by the windows. Our classroom is very beautiful.

流程图可以用来表示连续发生的一系列生理活动，以下流程中有错误的是（　　）

柳宗元在《封建论》中评价秦始皇废封建、行郡县说：“其为制，公之大者也，公天下之端自始皇。”郡县制为“公天下”之开端，主要体现在

对于低温诱导洋葱染色体数目变化的实验，不正确的描述是（  ）