下面关于隔离放大器，说法正确的是（）
A. 光电隔离放大器前后可以有电的连接
B. 光电隔离放大器前后不能共用电源，地线也不能接在一起
C. 光电耦合器体积大、工艺复杂、成本高
D. 电磁耦合体积小、使用寿命长、工作范围宽
Which of the following items would be patentable?
A distinctive way to produce a new cloth material
B. A textbook
C. The theory of relativity
D. A method that has the potential to increase output of a certain crop
In most cases, how long does a copyright lastin China?
A. 50 years
B. 17 years
C. 70 years
D. The author's life plus 50 years
The apple which identifies Apple Computers is an example of a
A. trademark
B. service mark
C. patent
D. Copyrighted work
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