在题30图所示系统中，已知输入信号http://img2.ph.126.net/mdupcFxlsOkyvuscItqFIQ==/6608438716887735791.png的频谱为http://img1.ph.126.net/Qb6wHkNlaXg7sGDatjXkdA==/6630691732724661829.png，输出http://img0.ph.126.net/An0Bj4yw02TeJvCw4kT1Qw==/6619515197025592278.png的频谱为( )http://nos.netease.com/edu-image/B9B387F861095FAB70D131ECC35E522F.bmp?imageView&type=jpeg&thumbnail=520x520&quality=100

1. 设题4图 (a)所示信号http://img2.ph.126.net/mdupcFxlsOkyvuscItqFIQ==/6608438716887735791.png的频谱http://img2.ph.126.net/-sGhI323Tr0yt1OwHRe20g==/6631437201607930994.png已知，则题4图 (b)所示信号http://img0.ph.126.net/An0Bj4yw02TeJvCw4kT1Qw==/6619515197025592278.png的频谱http://img2.ph.126.net/abbBaDCt2Vct2hhQQmrDAQ==/6631374529445217284.png 为 ( ) http://nos.netease.com/edu-image/46988664AF81F97D896D8324C9BB83F3.bmp?imageView&type=jpeg&thumbnail=520x520&quality=100