何谓退关箱？
在交谈中，每个人的发言，最好控制在多长时间内（）。 ["1到3分钟","3到5分钟","5到7分钟","7到10分钟"]
口岸查验包括（）、核查货证两种方式。 ["验证","换证","出证","抽查"]
Brushless generators are designed to operate without the use of（）. ["brushes","slip rings","commutators","all of the above"]
The reference datum used in determining the heights of land features on most charts is（）. ["mean sea level","mean high water","mean low water","half-tide level"]
什么是脱春化作用？
