To preserve the proper sound of c.
21. Under what condition is a consonant never doubled at the end of a word?
When immediately following a diphthong.—Webster.
22. When is C followed by K in spelling?
Words ending with the sound of k, and in which c follows the vowel.
23. Give some examples.
Back, black, fleck, etc.
24. Are there any exceptions?
There are, as sac, arc, etc.
25. Why is the word Humbugged spelt with two g's?