Usually, of course, we can only see the moon because it reflects sunlight.

通常我们只能看到月球是因为月球反射太阳光。

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Or course, short periods in the sunlight are also important for synthesizing vitamin D in the skin.

当然，晒一会太阳对于皮肤中维生素D的合成也是很重要的。

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Of course, balloons do not last as long as conventional power stations (each is estimated to have a working life of about a year). But the fuel (sunlight) is free.

当然，太阳能气球的寿命不会和传统发电厂一样，一颗太阳能气球的寿命大约一年，但是燃料(太阳能)却完全免费。

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