There are more than 20 amino acids that make up the biological polymers called proteins, so there are 20 choices of each amino acid at a position on a protein.

而那些称为蛋白质的生物活性多聚体,是由二十种不同氨基酸所构成,因此每个蛋白质的,任意位点都有多达二十种氨基酸可供选择

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So the library books are all optional background.

所以图书馆内的书,都是可供选择的背景资料

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On the other hand, we've seen n that if the size of a is n, that's to say, we have n elements to choose from, then the number of possible subsets is 2 to the n.

另一方面，我们看到,如果a集合的大小是，也就是说我们有n个元素可供选择，而可能的子集的元素,个数就是2的n次方。

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