سؤال اليوم — اختبار STEP

Read the passage and answer the question. (1) Elephants are known for their loud trumpeting calls, but much of their communication happens at a level humans cannot hear. In the 1980s, the researcher Katy Payne was visiting a zoo when she felt a strange throbbing in the air near the elephants, similar to the feeling of a deep note from a large pipe organ. She suspected that the animals were producing sounds too low for human ears. (2) Her suspicion proved correct. Using recording equipment, Payne and her colleagues discovered that elephants make deep rumbles below the range of human hearing, a type of sound known as infrasound. Because low-frequency sound waves lose less energy as they travel, these rumbles can carry over distances of several kilometers, especially in the cool, still air of early evening. (3) Later studies revealed that elephants may also detect these signals through the ground. Vibrations from a rumble travel through the soil, and elephants appear to sense them through sensitive areas in their feet and trunks. Researchers have observed elephants freezing, leaning forward and pressing their feet down when such vibrations arrive. (4) This long-distance communication is important for elephant society. Family groups, led by an older female, often spread out across large areas in search of food and water. Rumbles allow them to coordinate their movements, warn each other of danger and find mates. Each elephant also seems to have a distinctive voice that its relatives can recognize. (5) Understanding these signals may help protect elephants. Some conservation groups now place listening devices in forests to track elephant populations by their calls, a method that works even where the animals are hidden by thick trees. Question: As used in paragraph 2, the word "still" means