Is contamination still the most alarming assault on the environment or has another problem taken its place?

hi,   i believe the outlines that were supposed to be showing at the sentences, are not. could you highlight them somehow?

- "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."