单项选择题

在DNA纯度检测过程中，OD260/OD280＜1.6，说明（）

A．存在RNA污染
B．存在蛋白质或酚污染
C．溶液中有残存的盐
D．溶液中有残存的小分子杂质

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判断题

维生素、氨基酸、碱基都是微生物的生长因子。() A、正确 B、错误

单项选择题

Read the following paragraph and answer the question below. Chasing money in our 20s and 30s can lead to grief build up in our 40s as we spend more time reflecting and contemplating on the past, according to The Om Couple, self-improvement experts. We can't get back our time, and this lifetime is not a rehearsal. Spend time, not money. Failing to deepen connections to extended family members is something many people do as they are so busy with everything else in their lives — until we realize it's too late. Our nephews and nieces are suddenly all grown up, our grandparents passed away and our parents are beginning to forget things, says Milana Perepyolkina, author of Gypsy Energy Secret. If we kept the connection strong in our 30s, we would find a bigger circle of people who love and care about us in our 40s. Question: What does the author mean by saying Lifetime is not a rehearsal.

A．Life is short.
B．We can't go back our time.
C．Life goes without regret.
D．Life changes a lot.

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