21.  We can know a lot about this problem from its fallowing answer.

22.  The sellers in Taobao don't know enough about others' positions to price goods properly.

23.  Parents should know our learning condition, but not interfering.

24.  We'll know the goods only by the pictures which might have even been edited.

25.  We can know a person in any case.

26.  I didn't know anything about it.

27.  You can know the average prize, the quality and so on so that you won't get cheated because of the information asymmetry.

28.  They can know their stories and their areas that can enrich your life, and give you a Broad vision in solving problems, an optimistic character .

29.  We don't know the true quality of the commodity, as we don't know whether its appearance is the same as in the picutue on the website.

30.  Louis Victor de Broglie, every student major in physics must know him.