Many word processing programs include spell checker. It checks the spelling of every word in a (120) by looking up each word in its dictionary. If the word does not appear in the dictionary the user is (121) to possible misspelling and possible corrections are often (122) . Spell checker does not recognize unusual people names or specialized terms, but it will often allow you to create your own personal dictionary of specialized words you often use. Spell checker is a valuable aid to proofreading, but it can not catch the (123) of one correctly spelled word for another (such as form for from). Thus it does not (124) a document is free of spelling errors.

（123）处是（）

A.addition
B.condition
C.notation
D.substitution