A、To show the relationship between fearfulness and environment. B、To give examples of animals that aren’t fearful. C、To compare

We’ve been looking at fear from a biological perspective, and someone asked whether the tendency to be fearful is genetic. Well, some studies done with mice indicate that mammals do inherit fearfulness to some degree.
    In one study, for instance, a group of mice was placed in a brightly-lit open box with no hiding places. Some of the mice wandered around the box and didn’t appear to be bothered of being so exposed. But other mice didn’t move. They stayed up against one wall, which indicated that they were afraid. Well, when fearful mice, or you might say anxious mice like the ones who stayed in one place, when mice like these were bred with one another repeatedly, after about 12 generations or so, then all of the offspring showed similar signs of fearfulness. And even when a new-born mouse from this generation was raised by a mother and with other mice who were not fearful, that mouse still tended to be fearful as an adult.
    Now why is this? Well, it’s thought that specific genes in an animal’s body have an influence on anxious behaviour.
    These are genes that are associated with particular nerve-cell receptors in the brain. And the degree of overall fearfulness in the mammal seems to depend in large part on the presence or absence of these nerve-cell receptors. And this appears to apply to humans as well by the way. But while a tendency towards anxiety and fear may well be an inherited trait, the specific form that the fear takes has more to do with the individual’s environment. So a particular fear, like the fear of snakes or the fear of spiders, say, is not genetic, but the overall tendency to have fearful responses is.
Questions 23 to 25 are based on the passage you have just heard.
23. What is the talk mainly about?
24. How did some of the mice in the study demonstrate that they were afraid?
25. Why does the professor mention snakes and spiders?