题目

Long ago，some children were playing at seaside when they found a turtle(海龟). They began to beat(打) the turtle. Just at that time, a young man came and said to them, “Stop!” The children ran    16    quickly. The turtle was very thankful and said, “Thanks for your    17    . I truly would like to invite you to a wonderful palace now.” The young man rode on the back of the turtle and was taken to the secret palace in the sea. When he    18    the palace, he was very surprised and said to the turtle， “What a nice palace!” In order to thank him, the ki（2013•合肥模拟）已知正三角形A1B1C1的边长为1，作△A1B1C1的内切圆⊙O，再作⊙O的内接正三角形A2B2C2，继续作△A2B2C2的内切圆，…，如此作下去，则正三角形AnBnCn的边长为（ ）A．B．C．D．不能确定