单选题 How can researchers in "sleep laboratories" tell that the animals they are observing are asleep or not?

A、 They see if the animals respond to light and sound.
B、 They do this by observing changes in the animals' brain signals, breathing, heartbeat, and muscle activity.
C、 They see if the animals' eyes are closed.
D、 They can tell this by seeing if the animals lie down or not.

【正确答案】 B

【答案解析】从第三段“The answers come from…than when they appear to be awake．”可知，研究人员用一种叫做EEC的机器测量脑信号、呼吸、心跳和肌肉活动，以判断它们是否在睡觉，因为动物睡觉时和醒着时，测量结果是不一样的。