A、To reflect on their causes. B、To warn others against them. C、To make good sense of them. D、To avoid them in the future. D本题问为什

Stress is often depicted as negative, but research shows that moderate amounts of it can be beneficial for your brain and your body.
    First, the benefits for the brain. Studies have shown that short periods of stress can actually bolster cognitive functioning. Researchers discovered that placing rats in a stressful situation for just a few hours doubled the growth of new brain cells. The rats also did better on a memory test later on. Scientists think the same thing happens in humans. But how does stress improve memory? It’s simple. When your brain cells multiply, your memory can improve. Viewed from a biological perspective, this makes sense because animals that are better at remembering dangerous situations can avoid them in the future. If an animal encounters a predator and escapes, for example, it’s important to remember where and when that encounter happened. Experts assert that the same principle applies to humans.
    Now, let’s turn to how stress benefits the body. This may come as a surprise to laymen, but experts say that stress can keep you from getting sick. Scientists concede that chronic stress can make you more prone to illness, but research shows that short periods of stress can actually provide some protection against getting sick because it increases your immune functioning. One study shows that rats that experienced brief stress had a surge of immune cell response, which makes the immune system better prepared to fight illness. For humans, there is even evidence that experiencing stress before getting vaccinated could help make vaccines more effective.
    9.What did researchers discover about rats placed briefly in a stressful situation?
    10.Why do people tend to have clearer memories of dangerous situations they have encountered?
    11.What do scientists believe chronic stress can do?