In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.
Passage:

Never write down a phonetic __________ without fully understanding its meaning in every detail. A key to the various marks will be _____________ at the bottom of every page of the dictionary, and the student should refer to this ______________. In the front part of the dictionary there will also be found an _______________ of all possible sounds that any letter may have; and every sound that any letter may have may be indicated by a peculiar mark, __________ several letters may represent the same sound there are a variety of symbols for the same sound.