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In preparation for a large-scale tree-planting project, Thomasville has recorded

the location, species, and condition of every tree in its downtown area. The results will be compared with results of the last such tree census, taken 20 years ago. Planting trees primarily from the species that turns out to have the best record of survival will assure downtown Thomasville of an abundant tree population 20 years from now.

Which of the following, if true, most seriously undermines the claim that the tree-planting proposal will assure an abundant tree population?

A. Because of strict laws governing industrial development, environmental conditions in Thomasville are unlikely to become harsher for trees than they have been during the last 20 years.

B. Thomasville has reliable records to show which trees in existence 20 years ago were cut down to permit new construction downtown.

C. A number of trees from one of the species widespread in Thomasville 20 years ago reached the end of their natural life span in the intervening period.

D. Very few species of trees grow as well in urban conditions as in natural conditions.

E. A city with a high proportion of any one species of trees can have its tree population devastated by one outbreak of a disease.

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答案：

A 由于正在?

??強對工業(yè)的管制，樹木以后的生長環(huán)境不會比過去20年差，即樹木會獲得更好的生長環(huán)境。---加強。 B 有詳細的記錄，關(guān)于20年前存在，但后來為城區(qū)建設(shè)而被砍倒的樹木。---無關(guān)。因為無法判斷被砍倒的樹木可能有的生長狀況，無助于預(yù)計未來樹木生長狀況。 C 20年前長勢很好的樹，現(xiàn)在到了生長年齡的極限了，即將死亡。也就是說馬上會有一大片樹木死亡。那未來還應(yīng)該種這種樹嗎？還是種其他樹？無法推斷。 D 大部分的樹在城市中不如在自然環(huán)境下長得好。---無關(guān)。 E 如果城市樹木品種過于單一，那么這些樹木一旦爆發(fā)疾病，就會大批死亡。---正確。我們知道，同種植物之間的病蟲害是很容易傳染，難以阻絕的；而同一種病菌卻不容易在不同種植物間傳播。因此，如果該市單一地種植在過去20年長得不錯的樹種，假使疾病爆發(fā)，則很有可能倒下大半，則an abundant tree population 20 years from now計劃泡湯。