纳米材料表面微粒占微粒总数的比例极大，这是它有许多特殊性质的原因，假设某硼化镁纳米颗粒的大小和图形如图所示，则这种纳米颗粒的表面微粒数占总微粒数的百分数为()。（○镁原子，位于顶点和上下两个面心●硼原子，位于六棱柱的内部）
A．70%
B．22%
C．66.7%
D．33.3%
参考答案：A
解析：
这是一个纳米颗粒，其中的粒子不会与其他颗粒共用，因此所有的镁原子和硼原子都完全属于这个颗粒。该纳米颗粒表面共有镁原子14个，内部有硼原子6个，原子总数为20，所以镁原子占原子总数的百分比为
。故本题选A。
Sheila bought a new lunch bag for school. On the way to school, she looked inside. She hoped that a new lunch bag meant something new for lunch, 16 she found the same lunch as always---a sandwich. When the lunch bell rang. Sheila found her bag and carried it to the dining hall. She opened it and 17 two pieces of fried chicken, several strawberries, and an egg. “Wow!” Sheila said, “That’s what I call 18 !” The next morning, Sheila opened her lunch bag and looked inside. Sure enough, she found another 19 . But when she opened the bag at lunchtime in the dining hall, she discovered a piece of pizza, an orange, and a cake. “My lunch bag 20 be magic,” Sheila said. “There’s no such thing as magic,” her friend Douglas 21 . “There is,” Sheila said. “My mother 22 packs me a sandwich. But ever since I got this new lunch bag, the sandwich has changed into something I like.” “Maybe your mother’s packing 23 things for a change.” “No,” Sheila said. “I checked in the morning. The sandwich changes 24 .” “You are not the 25 one with a magic lunch bag,” Beatrice said, turning around from the table behind her. “I have one, too. See? This is my magic bag.” “Hey,” Sheila said. “My lunch bag really looks like yours.” “Mine is magic,” said Beatrice. “Every day my dad 26 leftovers(剩饭), but at lunchtime I find a sandwich---my favorite!” Sheila started 27 . “What’s so funny?” Beatrice asked. “That’s the sandwich my mother packs,” Sheila said. “We’ve been getting our lunch bags 28 up.” Sheila and 29 shared their lunches every day after that. Sheila loved making new discoveries each time she opened her lunch bag. But her best discovery was magically finding a new 30 .
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