根据所给的首字母填写单词。

     I am a middle s    1   student now. I h   2   four new classmates. T  3   are Lucy, Sally, Alan a   4   Carl.

Lucy and Sally a   5   girls. Alan and Carl are boys. Lucy is tall. She is slim, t   6   . Sally is not tall. She is

small and thin. Alan is Lucy's cousin. He i   7   big and strong. He is v   8   helpful. Carl is a short boy. He is f   9   England. All m   10   new classmates are polite. We are good friends.

当双极性基带信号码元独立等概时，其2PSK信号的功率谱密度为（）。

A.连续谱

B.离散谱

C.连续谱和离散谱

D.直流