Reference books are not designed to be read from the first page to the last but rather to be consulted to answer the questions and provide both general and specific pieces of information.

One of the most (31) _____ -used reference books is a dictionary, which provides information about words. It lists meanings and spellings, tells how a word is pronounced, gives (32) _____ of how it is used, may reveal its origins and also lists synonyms and antonyms. To help you find the words faster, there are guide words at the top of each page showing the first and last words on that page - and of course it (33) _____ to know the alphabet!

There may be numerous special sections at the back with facts about famous people and places, lists of dates and scientific names, etc. There is usually a section at the front (34) _____ how to use the dictionary, which includes the special abbreviations or signs.

An atlas is also a reference book and it contains charts, tables and geographical facts, as well as maps. Political maps locate countries and cities, physical maps show the formation of the land with its mountains and valleys, and economic maps show industries and agriculture. To find a specific place, you need to look in the (35) _____ at the back of the atlas and the exact position on the map. There are numerous map symbols that you need to know in order to be able to read a map-almost like a special language-and these are explained at the front of the atlas.

On returning to the surface, the men said that they had experienced no difficulty in breathing and had (34)______ many interesting scientific observations.