[单项选择]The Japanese Macaca monkey has been studied in the wild for over 50 years. In 1952, on one small Japanese island, scientists dropped some (21) in the dirt for the monkeys. The monkeys liked their taste, but they found the dirt (22) .
One clever 18-month-old monkey found she could (23) the problem by washing the sweet potatoes in a nearby river. She taught this to her mother. Her (24) also learned this new way and they taught their mothers too.
All the younger monkeys (25) learned to wash the dirty sweet potatoes to make them (26) to eat. But many old monkeys found it very hard to learn this and still ate the (27) sweet potatoes.
Then something very strange (28) In the autumn of 1958, scientists found that the monkeys on other nearby (29) began washing their sweet potatoes too. Scientists still don’t fully understand how this knowledge was (30) from one island to another.
Ⅱ
A. trees
B. islands
C. beaches
D. rivers

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