3. The United States Postal Service charges for the service of Informed Delivery from the users.
A、正确
B、错误

不定项选择The idea of a free trial period is often used to sell computer games, memebership of a club,ect.

多项选择题装配单元用到了 下列哪些传感器？（ ） A、磁性开关 B、光电传感器 C、光电编码器 D、原点开关