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C is sounded hard like k before a, o, or u; or before any consonant except h; or at the end of a syllable, if not followed by i or e: eg, cake, Coke, cuckoo; clock; optic
C is sounded soft like s before e, i, or y: eg, concentrate, reconcile, cynic
Monosyllables ending with the sound of k, and in which c follows the vowel, usually have k added after the c: eg, quack, check, trick, mock, tuck
Words of more than one syllable ending with the sound of k, and in which the c follows a vowel or vowels other than i or ia, usually have k added after the c: eg, bullock, hammock, haystack
Words of more than one syllable ending with the sound of k, and in which c follows i or ia, are usually written without the final k: eg, picnic, athletic, prosaic, maniac, cardiac
Source: Eldridge's Business Speller
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