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

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.）

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。