SDH的线路码型为(),2M信号的码型为(),140M信号码型为()
参考答案:扰码加NRZ,HDB3,CMI
If I hadn’t stood under the ladder to catch you when you fell, you _____ now.
A.didn’t smile
B.won’t smile
C.wouldn’t be smiling
D.couldn’t have smiled.
小孢子母细胞进行减数分裂前,花粉囊壁一般由表皮、()、()和()组成。花粉成熟时,药壁只留下表皮和()