Because it takes three units where there's only four choices at each position to have at least 20 unique combinations.

因为每个位点上仅有四种可能性,只有采用三个碱基一组,才能取得至少二十种不同的组合方式

耶鲁公开课 - 生物医学工程探索课程节选

How many combinations of codons are there if there's three bases and four possibilities at each base?

那密码子的组合方式到底有几种呢,三个位点,每个位点四种选择

耶鲁公开课 - 生物医学工程探索课程节选

Okay, so now we've seen four different combinations.

目前为止我们看到了四种组合

耶鲁公开课 - 博弈论课程节选