Alife spent making mistakes is not only more honorable but more useful than a life spent doing nothing

i would say the correct answer is not concrete.

- "the basic difference between keywords and subjects is that subjects (or "subject headings") have been predetermined, while keywords have not. a keyword is an easy place to start because you can use any word that seems to fit your search; this is called "natural language" searching."

i think it is c. good luck!