Perhaps you should try some spelling variations:
20th century » 18th century
ref history » reef history, brief history, bief history
11 germany » 1918 germany, 7 germany, 8 germany
11 german » 1 german
90 20th » 20 20th, 94 20th, 1990 20th

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: %22agriculture AND state %((11 germany) OR (11 german)) %ref history %90 20th century%22.
• The words AND, OR and NOT may confuse the search; try adding quotes: "%22agriculture and state %((11 germany) OR (11 german)) %ref history %90 20th century%22".
• Adding a wildcard symbol may retrieve word variants: %22agriculture and state %((11 germany) OR (11 german)) %ref history %90 20th century%22*.