有一段mRNA,含有2000个碱基,其中A有400个,U有500个。由此mRNA逆转录产生的含有4000个碱基的双链DNA中A和G分别有(　　)

孔子曰“非礼勿视，非礼勿听，非礼勿言，”“重礼敬德”的中华礼仪传统在历史长河中沉淀下来。孔子所说的“礼”在当时所起的主要作用是（   ）

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A

      People in different places eat different things.

　　In South China people like to eat rice. Sometimes they eat it twice or three times a day. They usually eat it with fish, meat and vegetables. It tastes very delicious.

      Japanese like to eat rice, too. They also eat a lot of fish. Sometimes they eat raw (生的) fish. It sounds strange but smells very fresh.

　　In western countries like Britain, Australia and the USA, the most important food is bread or potatoes. People usually make their bread at home. And they cook potatoes in different ways. In England the most popular food is fish and chips. Sometimes people eat this food at ho me, in their workplace, in the park or even on the street. People call it ”take­away” food.

　　In China today, there are also much ”take­away” food, and most children like to eat them.

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Dear Sir or Madam,

Yours faithfully,

John

在2Mg+O₂2MgO 反应中，各反应物、生成物之间的质量比是（　　）