1. (2021·海南模拟) 阅读理解

Researchers from China and the United States have found what makes ginkgo (银杏) trees live for more than 1, 000 years, according to a newspaper report. Researchers from Beijing Forestry University, the University of North Texas and some other Chinese research institutions reported in the U. S. magazine Proceedings of National Academy of Sciences that they analyzed 34 ginkgo trees and divided them into three groups according to their ages. Compared with adult trees, the ancient trees, aging from 193 to 667 years, have thinner cambium (形成层) cell layers(细胞层). Cambium cells are layers of stem cells between the inside wood and outside bark changing into other tissue.

The old trees have a higher level of IAA, a plant growth regulator and growth-stimulating hormone, and a lower level of ABA, a growth-resisting hormone. However, the average cross-sectional area of trees at breast height in the ancient trees group has been increasing with aging, which shows the continuous cambial cells division.

Little difference was found when it comes to their ability to photosynthesize(光合作用), grow leaves, grow seeds or resist disease compared to younger trees, showing that the ancient trees group are actually in a healthy and mature state.

Lin Jinxing of Beijing Forestry University and an author of the study said that trees usually die from outside stress like pests. They found disease resistance-associated genes kept high expression in old trees, along with genes associated with the synthesis(合成) of protective compounds, showing that the old trees do not lose their ability to defend against outside stressors. The researchers concluded that the longevity(寿命)of ginkgo trees is a balance between the growing and aging processes, rather than being regulated by a single gene.

1. （1） What are special about cambium cell layers of ancient trees?

   A . They are old enough. B . They are a little thinner. C . They exist in the internal wood. D . They can change into other tissues.
3. （2） What will happen if a tree has a high level of ABA?

   A . It will stop growing. B . It will have more tissues. C . It will grow slowly. D . It will die soon.
5. （3） Compared with young trees, ancient ginkgo trees_________.

   A . don't have a lot of leaves B . are not in a healthy state at all C . don't have a strong ability to fight against disease D . don't have much difference in growing seeds
7. （4） What is the last paragraph mainly about?

   A . The outside factors of most trees' death. B . The reason for ginkgo trees' long life. C . The trees' ability to defend against stressors. D . The relation between genes and trees 'life.