单项选择题

Summer has come, making us think of all kinds of nice things, such as holidays in the country.
The days are long and the nights are short, for the sun gets up early and goes to bed late in sunny June. It is one of the best and brightest months in the year. Birds sing all the day.
As night falls, the birds stop their song, one after another. However, they do not all sleep. When other birds are silent, a soft (轻柔) cry comes, then, after a few moments, louder. It is the nightingale (夜莺) singing his beautiful song.
The nightingale not only sings at night. On almost every warm day in early June, the little brown bird sings away on the roads and trees, but because all the other birds are also singing at the same time, we do not hear his voice so much. It is only at night, that we hear the nightingale at his best.
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