我们能看清一个物体是因为（     ）

根据短文内容，选择正确答案。

D

    Astronauts (宇航员) face many problems when living in space stations. One of them is food. People on Earth put together all the meals first and then send these meals to space stations by space shuttle. Because the food has to last a long time (sometimes up to three months), people have to store a lot of it in cans. The space station does not have a fridge, but it has a cool room to keep fruit and vegetables fresh. Astronauts also eat many other foods such as dried meat that do not need special care.

    Without the help of gravity sitting down to eat can be difficult. Astronauts sometimes have to fix themselves to the wall while eating. They also have to be very careful so that food does not float away.

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驿路梨花（节选）

       ①我们正在劳动，突然梨树丛中闪出了一群哈尼小姑娘。走在前边的约莫十四五岁，红润的脸上有两道弯弯的修长的眉毛和一对晶莹的大眼睛。我想：“她一定是梨花。”

       ②瑶族老人立即走到她们面前，深深弯下腰去，行了个大礼，吓得小姑娘们像小雀似的蹦开了，接着就哈哈大笑起来：“老爷爷，你给我们行这样大的礼，不怕折损我们吗？”老人严肃地说：“我感谢你们盖了这间小草房。”

       ③为头的那个小姑娘赶紧插手：“不要谢我们！不要谢我们！房子是解放军叔叔盖的。”

       ④接着，小姑娘向我们讲述了房子的来历。十多年前，有一队解放军路过这里，在树林里过夜，半夜淋了大雨。他们想，这里要有一间给过路人避风雨的小屋就好了，第二天早上就砍树割草盖起了房子。她姐姐恰好过这边山上来抬菌子，好奇地问解放军叔叔：“你们要在这里长住？”解放军说：“不，我们是为了方便过路人。是雷锋同志教我们这样做的。”她姐姐很受感动。从那以后，常常趁砍柴、拾苗子、找草药的机会来照料这小茅屋。

       ⑤原来她还不是梨花。我问：“梨花呢？”

       ⑥“前几年出嫁到山那边了。”

       ⑦不用说，姐姐出嫁后，是小姑娘接过任务，常来照管这小茅屋。我望着这群充满朝气的哈尼小姑娘和那洁白的梨花，不由得想起了一句诗：“驿路梨花处处开”。