The union ensured that a worker could not be laid off arbitrarily by an employer.

　　20. The author of the passage mentions particular employers primarily in order to

　　 suggest that occupational unions found some employers difficult to satisfy

　　 indicate that the occupational unions served some employers but not others

　　 emphasize the unique focus of occupational unionism

　　 accentuate the hostility of some employers toward occupational unionism

　　 point out a weakness of worksite unionism

　　In prehistoric times brachiopods of marine invertebrates with bivalve shells within which is a pair of arms bearing tentacles by which a current of water is made to bring microscopic food to the mouth) were one of the most abundant and diverse forms of life on Earth: more than 30,000 species of this clamlike creature have been cataloged from fossil records. Today brachiopods are not as numerous, and existing species are not well studied, partly because neither the animals fleshy inner tissue nor its shell has any commercial value. Moreover, in contrast to the greater diversity of the extinct species, the approximately 300 known surviving species are relatively uniform in appearance. Many zoologists have interpreted this as a sign that the animal has been unable to compete successfully with other marine organisms in the evolutionary struggle.

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