按信号的类型来分类，信道可分为（）。
A、模拟信道；
B、频分制信道；
C、时分制信道；
D、数字信道。
参考答案：A, D
Lucy does her homework very ________. She is ________ than Lily.
A．carefully, much more careful
B．carefully, much carefuller
C．careful, many more careful
D．careful, a little carefuler
仪器从仪器箱中取出后应（）。
A.盖好仪器箱，立放
B.盖好仪器箱，平放
C.人可以坐在仪器箱上
D.将其他的辅助小工具放入仪器箱中，防止丢失
