题目

完形填空 Why did the orangutan（猩猩）cross the road? That's not the start of a joke, but a question being asked by scientists studying these brilliant apes. Brent Loken, who runs a group called Integrated Conservation, used camera traps to research into orangutans. That group protects 1 animals in Borneo, the largest island in Asia. Camera traps work by taking a quick photo, when something moves in front of them. In Borneo, scientists use the photos to 2 what the rare orangutans do when people aren't 3? Scientists have been studying orangutans in the rainforests for decades. And thei与声音传播速度有关的是A．声音的音调B．声音的响度C．声音的频率　　　D．传播声音的物质