简述直放站的传输方式是透明传输，请解释透明传输的含义。
参考答案：
输出信号频率与输入信号频率相同，透明信道。
下列运算正确的是 [ ]
A．6a+2a=8a2
B．a2÷a2=0
C．a﹣（a﹣3）=﹣3
D．a﹣1a2=a
—How do you like China?
—Great changes _______ since I _______ here 8 years ago.
A. have taken place; came B. took place; have come C. took place; came
