Engineers took those situations into account when building out their concept drone. They recorded themselves screaming, tapping and creating other noises that might be a sign of people in trouble. Then, they analyzed each sound frequency to find common signatures and used those to train artificial intelligence software. They also worked to remove the noise created by the drone and other environmental sounds.

Once the software part was complete, the team placed tiny digital microphones under the drone and used signal processing techniques that enabled them to track where human noises are coming from. The microphones also enhanced the volume and clarity of the speech. So far, they have conducted successful open field experiments, finding that the drone can estimate a victim’s location within a few seconds of picking up sound.

What does the underlined word “signatures” in paragraph 1 refer to?

A．Effects.

B．Symbols.

C．Features.

D．Situations.