题目内容
According to the passage, ______.
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The "Karat" marking on jewelry tells you what proportion of gold is mixed with other metals. If 14 parts of gold are mixed with 10 parts of base metal, the combination is called 14 Karat (14K) gold. The higher the Karat rating, the higher the proportion of gold in the object. The lowest Karat gold that can be marketed in the United States is 10-Karat gold. Jewelry does not have to be marked with its Karat quality, but most of it is. If there is a Karat quality mark, next to it must be the U.S. registered trademark of the person or company that will stand behind the mark, as required by the National Gold and Silver Stamping Act.
If a ring is stamped 24K, it has ______.
If the jewelry is marked 14 parts of gold mixed with 10 parts of base metal, it will always bear ______ mark.
Gold which is 10 Karats in proportion ______ in the U.S.
Since life began eons ago, thousands of creatures have come and gone. Some, such as the dinosaurs, became extinct due to naturally changing ecologic conditions. More recent threats to life forms are humans and their activities. Man has drained marshes, burned prairies, dammed and diverted rivers. Some of the more recent casualties of man's expansion have been the dodo, great auk, passenger pigeon, Irish elk, and Steller's sea cow. Sadly, we can no longer attribute the increasing decline in our wild animals and plant species to "natural" processes. Many species are dying out because of exploitation, habitat alteration or destruction, pollution, or the introduction of new species of plants and animals to an area. As mandated by Congress, protecting endangered species, and restoring them to the point where their existence is no longer jeopardized, is the primary objective of the U.S. Fish and Wildlife Service's Endangered Species Program.
Which of the following is a form. of man's habitat alteration？
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