题目内容：
根据下面内容，回答题： Thousands of years 21 humans began the process of domesticating the dog and shaping what"being a dog" really means. Through careful selection and breeding, an astonishing variety of dog breeds 22 been created. Desirable traits have been selected for in various breeds that are of a benefit 23 humans. There are some traits, however, that quickly become undesirable when expressed too frequently. Barking is a(an) 24 of a natural behavior that is encouraged in terms of guarding behavior, but becomes a problem when the behavior is produced in excess. A recent health insurance investigation revealed 25 the sound of a continually barking dog was cited as the most disruptive and stress inducing 26 for humans.
Barking, in addition to whining, howling and growling, is a dog's 27 means of communication. Barking is characterized by a series of short, sharp sounds, that tend to vary 28 in tone or pitch. A dog's bark 29 signify territorial protection, exertion of dominance, or expression of some need.30, barking is "a means of communication triggered by a state of excitement. " Being a natural trait, barking is not considered a behavioral problem,31 it is produced in excess.
The key to solving the problem of excess barking in your dog begins with an understanding of what is causing 32 behavior. Once you have determined a cause,33 have a greater chance of choosing the most effective solution (e. g., more exercise) or behavioral modification. Modifying such an instinctive and natural behavior 34 barking can be difficult, and may require considerablepatience, time, and hard work.35, however, are possible, and worth the effort.
A．since
B．before
C．ago
D．later
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