One day, in an English class, the students are having their lessons. Their teacher, a young woman is very happy. She sings some nice songs with them and then she goes to the blackboard and writes “SMILES” on it. “This is the longest（最长）word in English,” she says to the class.
   A girl stands up and says, “It has only six letters. How can it be the longest word in English?”The teacher smiles and says, “Oh, look! There is a mile（英里）between the first letter and the last one.”

求下面圆的周长．

如图所示，甲、乙、丙、丁是不同的声音先后输入到同一示波器上所显示的波形图．则下面说法中正确的是（　　）

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