阅读理解。
Animals can move from place to place, but plants cannot. When an animal is under attack, it can run
away or fight back. Plants certainly cannot run away, and they lack teeth and claws. But plants can
defend (保护) themselves by using both physical and chemical means.
Some plants have their own ways to keep animals away. For example, the leaves of the holly plant
have sharp spines (刺) that discourage grass-eating animals. Holly leaves on lower branches have more
spines than leaves on upper branches. This is because the lower leaves are easier for most animals to
reach.
Some plants, such as the oak tree, have thick and hard leaves that are difficult for animals to eat. Some
grasses may contain a sandy material; eating such grasses wears down the animal's teeth.
Many plants also have chemical defenses. Some plants produce chemicals that taste bitter or cause an
unpleasant reaction. Some plants may fight against an attack by increasing the production of these chemicals. When a caterpillar (毛虫) bites a tobacco leaf, the leaf produces a chemical messenger. This messenger sends to the roots the information to produce more nicotine. The higher levels of nicotine discourage the
caterpillar.
Many plants depend on both physical and chemical defenses. A certain plant in China, for instance,
has prickly (多刺的) leaves, and each prickle contains poisonous venom (毒液) A single experience with
this kind of plant will teach an animal to stay away from it in the future.
1. To defend themselves, oak trees use________.
A. chemical means
B. physical means
C. bitter chemicals
D. sandy materials
2. How does tobacco protect itself against an attack from a caterpillar?
A. Its leaves fight against the attack by physical means.
B. Its roots send a messenger to discourage the caterpillar.
C. Its roots increase the production of nicotine when it is attacked.
D. Its leaves produce poisonous sand to drive the caterpillar away.
3. What would be the best title for this passage?
A. Plants and Animals
B. How Plants Defend Themselves
C. Attacks and Defenses
D. How Animals Eat Plant Leaves
和世界人均占有量的1/30。北京市水利专家分析认为，近几年京城缺水年平均在4亿立方米左右，预计2007年缺水可能比这一平均值增加一倍。依此速度，到2050年，可能较这一平均值翻三番。
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