B．Human knowledge.

C．Every single person.

D．A common cause.

I was asked in an email about learning idioms in a foreign language.Maybe I am different from most people，but I do not bother with them.To me they are kind of dessert of language learning.They come at the end of a meal.Once you have filled up with the solid fare-the key words and phrases that are used in a variety of situations，then you can add a few idioms to spice up things.And by that time you will be able to pick them up naturally anyway.

I find that many learners have an obsession(困扰) with slang and idioms，as if repeating some very colloquial expressions is going to upgrade how they sound in a language.In fact it is the opposite.Idioms are difficult to use correctly.They can easily sound very strange coming from someone who obviously does not have a good sense of the language.

Yes，occasionally you hear idioms or slang and do not understand.But，in my experience，there are always situations where you do not understand.It is best to focus on the most important words，what they mean and how they are used.Certainly，it is best for a nonnative speaker to stick to standard speech.

Some teachers even try to teach the "real language"．Then you hear nonnative speakers saying things like "I gonna" ，"You wanna" etc.I just cringe(厌烦)．

You will learn all the idioms you need just by exposing yourself to the language.In general，you can save any phrase that you find interesting，and if that includes idioms，go to it.But I would not make any special effort to go after idioms.I，in fact，avoid them.If I come across something in Russian that strikes me as a particularly slangy or idiomatic use of words，I just leave it aside; do not even bother trying to learn it.If it keeps on appearing，then I will learn it.But I would certainly be reluctant(勉强的) to try to use it.

4．In the writer's opinion，when one learns a new language，one should learn its idioms