题干
_____ mobile phone is very convenient, school children are not allowed to own one.
A：If
B：While
C：Because
D：Whe
答案(点此获取答案解析）
B
、 
两球用细线悬挂于天花板上且静止不动．两球间是一个轻质弹簧，如果突然剪断悬线，则在剪断悬线瞬间 