Most commentaries have something to do with description, explanation, or opinion. But the three are not always present in equal proportions.（ Some forms of written commentary for instance, by providing the supplementary information which will enable a text to be more fully understood, set out purely to explain.）
【正确答案】：比如，有些评述文章完全是说明文，它们为读者提供补充信息，使读者完整地理解文章。
【题目解析】：原句很长，翻译时需要打破英语的句子结构，根据意群拆成短句来翻译。这里，我们把“for instance”提到句首，把“written commentary”作为主语，然后把by后面的长句拆成两个意群来翻译，即“为读者提供补充信息”和“使读者完整地理解文章”。
