题目

If you walk slowly through downtown Helsinki during the day, taking in the splendid 19 th century buildings, white boats and noise of passing trams (有轨电车), you will start to understand why it is called a city of two colors: white and blue. The sea is always present in Helsinki. When you take a walk over the great open space of the central square, you will hear seabirds screaming. When you take the tram, suddenly and unexpectedly, you are faced with a calm, shining blue sea. You may notice that people in Helsinki do not rush about as in other cities. Instead, they walk along the roads, 依据卢瑟福的原子行星模型理论，在原子中绕核高速旋转的是 [     ]A．核子 　　B．电子　　 C．质子　　 D．中子