Many students find it hard to ______ the rules in this school.[ ]

The importance ______ a national day andhow it is celebrated are very different from country to country

Obviously, the advantage we take of solar energy is ________ enough.We have to work still harder.

I admire Stephen Hawking greatly，whose________about black holes has a great effect on astronomy.

The politician has been warned that he would be getting into great trouble ________stick to his standpoint.