先通读下面的短文，掌握其大意，然后从下面四个答案中，选出可以填入空白处的最佳选项。

The world of out-of-doors is full of secrets．And they are so interesting that quite a lot of people are busy studying them．All around us are birds, animals, trees and flowers．The facts about how they 　　1　　 and grow are as interesting as anything could be．Do you know that one of the greatest presidents of the USA spent hours and hours 　　2　　 birds? A businessman who lives near New York city became so 　　3　　 in insects that he began to collect them．He now has more than one thousand different kinds carefully 　　4　　 in glass boxes．Come then with me, and I will help you find some of nature's secrets．Let us go 　　5　　 through the woods and fields．Here we shall find how a rabbit tells the other rabbits that there is danger．We shall 　　6　　 a mother bear and her young ones as they look for 　　7　　 and get ready for winter sleep．We shall watch bees dancing in the air to let other bees know 　　8　　 they can find food．I will show you many other interesting things, but the thing that I can 　　9　　 you is to keep your eyes and ears 　　10　　 when you go out of doors．Nature tells her secrets only to people who look and listen carefully．

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(选择合适的单词或短语并用其适当形式填空)

　　in the open air, all types of, watch, China, long, a number of

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