C1061 Exchange of hostages

Li Qingyu opened his mouth slightly and let out a long soundless whistle.

What he released was an ultrasound, which ordinary people could not hear, but the Sky Patrol General Black Wing Roc could, and the Black Wing Roc could, too, emit the sound wave, which Li Qingyu could easily detect.

This was a new form of communication that had been developed just a while ago between a man and a peng