您要查找的是不是：
- neecessary cenditions 必要条件
- It is neecessary to place one eye on the small eyepiece, so it is not posible to observe other aircraft which would otherwise be seen within the normal field of vision. 它要求把一只眼睛靠近微小的目镜，这样就无法看到那些出现在正常视野中的飞机。
- CENDIT Centre for Development of Instructional Technology 教学技术开发中心
