Classification is a useful ______ to the organization of knowledge in any field.

. To our ______, the result of the experiment is much better than we expected.

It was reported that the government official expressed the deepest ______ to the families of those killed and injured in this attack.

The       of surfing the Internet is how you can tell whether the information is true or not.

He is ______ as a leader but he hasn't ______ in teaching.