下列语句中，不正确的个数是（　　）

①直径是弦；②弧是半圆；③长度相等的弧是等弧；④经过圆内一定点可以作无数条直径．

读某区域图，结合所学知识完成下列各题。

根据短文理解，回答问题

    Many students in the USA always take the school bus to school every day. Parents think taking the school bus is very safe. But sometimes it is also dangerous (危险的). Some students die (死亡) because they don't follow the school bus rules. Here are some school bus rules:

◆Arrive at the bus station early.

◆Wait for the bus at the bus station.

◆Don't play around (在周围) the bus.

◆Don't run everywhere at the bus station.

◆Don't walk or run in the bus when it is going.

◆Don't try to leave the bus before it stops.

These rules are good for students. Remember them and follow them. So you can keep yourself safe.

下列装置或操作能达到实验目的的是(     )