A dog is getting aggressive .A dog has discovered something .None of the above choices .A dog is ready to attack .
What may be a fact about this situation ?
He 's crouched down , close to the ground , sniffing it cautiously . Then he lifts his nose to the air once more , turns his head either way , and inhales deeply . Then he lets out a small growl and turns to the other three while on all fours .