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.
The abuse of phrases and the misuse of words rife among us can be checked __________ diligent exercises in good English, such as this book provides. __________ others to be found in different volumes by the same author, will serve to correct careless diction and slovenly speech, and lead to the art of speaking and writing correctly; for, after all, __________ in the use of words is more a matter of habit than of theory, and once it is acquired it becomes just as easy to speak or to write good English __________ bad English.

For the first blank: "can be checked __________ diligent exercises"

1. A
   by
2. B
   to
3. C
   at
4. D
   so