What are the features of these expressions
选项：

A:An idiom is an expression that has a special meaning that is different from the meaning of the individual words.
B:Idioms in one language are hard for learners to understand or even impossible to guess.
C:Idioms grow out of events and usage within the specific culture, which is why learning the cultural behavior is as important – and maybe more important in some instances – as (than) learning the words if we are to achieve truly effective communication.