21.Among the trees left over after the planting is done

　　there must be

　　（A）at least one maple

　　（B）at least one red oak

　　（C）at least one sycamore

　　（D）at most one maple

　　（E）at most one red oak

　　22.If maples are planted， the side of the street that has

　　four trees must have

　　（A）red oaks in positions 1 and 7

　　（B）red oaks in positions 3 and 5

　　（C）sycamores in positions 1 and 3

　　（D）sycamores in positions 1 and 7

　　（E）sycamores in positions 3 and 5

　　23.A list of the fifteen operas most frequently performed

　　in recent times includes no works by the nineteenth-

　　century German composer Richard Wagner. Although

　　music producers tend to produce what audiences want，

　　relative infrequency of performance probably does

　　not indicate lack of popularity in Wagner's case， since

　　Wagner's operas are notoriously expensive to perform

　　on stage.

　　Which of the following， if true， most strongly

　　supports the conclusion of the argument above？

　　（A）The list of most frequently performed operas

　　does not include operas produced by small

　　amateur groups.

　　（B）Some opera companies are backed by patrons

　　who are willing to commit large sums of

　　money in order to enjoy lavish productions.

　　（C）All of the fifteen most frequently performed

　　operas of recent times are works that have

　　been popular for at least 75 years.

　　（D）More recordings have been produced recently of

　　the works of Wagner than of the works of any

　　other composer of opera.

　　（E）Operatic works of all kinds have been increasing

　　in popularity in recent years.

　　24.The bodies of dwarf individuals of mammalian

　　species are generally smaller in relation to those of

　　nondwarf individuals than are the teeth of the dwarf

　　individuals in relation to those of the nondwarf indi-

　　viduals. Fragmentary skeletal remains of an adult

　　dwarf woolly mammoth were recently found. The teeth

　　are three-fourths the size of the teeth of an average

　　adult nondwarf woolly mammoth.

　　The statements above， if true， most strongly support

　　which of the following？

　　（A）The body of the dwarf woolly mammoth was

　　less than three-fourths the size of the body of

　　an average adult nondwarf woolly mammoth.

　　（B）None of the teeth of the dwarf woolly mammoth

　　that were recently discovered was as large as

　　any of the teeth of nondwarf woolly mammoths

　　that have been discovered.

　　（C）The teeth of most adult dwarf individuals of

　　mammalian species are three- fourths the size of

　　the teeth of the adult nondwarf individuals of

　　the same species.

　　（D）Dwarf woolly mammoths had the same number

　　of teeth as did nondwarf woolly mammoths.

　　（E）Dwarf individuals of most mammalian species

　　are generally no more than three-fourths the

　　size of the adult nondwarf individuals of those

　　species.