题目内容：

When a caterpillar (毛虫)pretends to be a snake to keep off potential predators (捕食者)，it should probably expect to be treated like one. This is exactly what happened in Costa Rica earlier this year, when researchers witnessed a hummingbird defending its nest from what it interpreted to be a snake, but was actually a caterpillar, which was feeding on a leaf above the nest.

These caterpillars resemble flat dried leaves as adults. They can expand the top of their heads to expose a pair of eyespots that discourage potential predators. When disturbed, they raise their head up and move from side to side, increasing the snake-like appearance. In particular they resemble a green parrot snake, known to feed on nesting birds.

Hummingbirds have a few styles of flying: visiting flowers, feeding on some tiny insects, chasing each other， and mating territory (领土)display flights. Mobbing (围攻)behavior directed against a threat to their nest is much less common but easy to recognize if you know their flight behaviors.

The interaction of the caterpillar and the humming birds took place in Costa Rica. The comings and goings of the female hummingbird around its nest may have disturbed the caterpillar, causing it to expose its eyespots, which in turn caused the hummingbird to defend its nest using what is referred to as ‘mobbing behavior’ by birds.

The caterpillar was unable to feed during the 26-minutes of nearly continuous attacks. Most of the bird’s movements were cautious, but included quick attacks to peck or bite the eyespots.

It was difficult for either the bird or caterpillar to withdraw from the standoff (僵局)，with the hummingbird protecting its nest and the caterpillar just trying to finish its leafy meal. Eventually the caterpillar gave up on eating and crawled away while still under attack, and the hummingbird continued normal nesting behavior.

When birds exhibit this mobbing behavior targeting snaked eyes, it often ends with snakes being killed by repeated bites and pecks near the head. As for creatures that imitate snakes to protect themselves from being eaten, can they in turn protect themselves? Because the hummingbird behavior was typical anti-snake behavior, it can be considered replicable (可复制的).

Future studies of this behavior can be conducted using a tiny, caterpillar robot. It should be remote controlled, light enough to attach to a leaf or stem, and wireless. With such a robot, researchers could vary the eye-like nature and contrast of spots on the head of the robot to test various responses of nest-defending birds. A study like this could definitively test the effects of eye-like versus other imitating patterning for exciting the defensive attacks.

And, it turns out that the caterpillar feeds on a leaf on the same plant, as far away from the nest as possible!

1.What do we know about caterpillars and hummingbirds from the passage?

A. They belong to the same species.

B. They are natural enemies of each other.

C. They are insects and birds respectively.

D. They compete against each other for territory.

2.Hummingbirds will show their mobbing behaviors when visitors____.

A. are picking flowers    B. are chasing each other

C. interrupt their eating leaves    D. set foot on their territories

3.Why is neither the hummingbird nor the caterpillar willing to withdraw at first?

A. Because the hummingbird is trying to finish its leafy meal.

B. Because the caterpillar is protecting the hummingbird’s nest.

C. Because they behave in this way out of their own interest.

D. Because there is a green parrot snake watching them.

4.A tiny caterpillar robot can be used to____.

A. imitate the anti-snake behavior

B. test nest-defending birds’ responses

C. take control of the snake’s behavior

D. guard against the defensive attacks