问题 选择题

“天苍苍、野茫芒,风吹草低见牛羊。”描写了一幅壮美的草原风光。该草原生态系统中主要应控制什么生物的数量,才能保持生态系统的平衡,使壮美的风光依旧

A.草

B.牛和羊

C.狼

D.蛇

答案

答案:B

题目分析:生态系统中各种生物的数量和所占的比例是相对稳定的.这种平衡是一种动态平衡,之所以会出现这种平衡是因为生态系统具有一定的自我调节能力,由于这种能力与生态系统中生物的种类和数量有关,生物的种类和数量越多,营养结构越复杂,这种能力就越强,反之,就越弱;在生态系统中,物质能量是沿着食物链、食物网流动的,并逐级减少,每一个营养级大约减少20%,能量的传递效率约为80%,营养级越多,在能量流动中消耗的能量就越多,但这种自动调节能力有一定限度,如果外界干扰超过了这个限度,生态系统就会遭到破坏。“天苍苍,野茫茫,风吹草低见牛羊,”描写了一幅壮美的草原风光,该草原生态系统中应该控制牛、羊的数量,才能保持生态系统的平衡,让壮美的风光依旧,如果牛、羊数量增加会使草的数量减少,草减少导致牛、羊由于食物不足被饿死,数量减少,直到牛、羊和草的数量达到平衡为止,故B符合题意。

单项选择题
阅读理解

阅读理解。

     Early in the 16th century men were trying to reach Asia by travelling west from Europe. In order to

find Asia they had to find a way past South Africa. The man who finally found the way from the Atlantic

Ocean to the Pacific was Ferdinand Magellan.

     Magellan sailed from Seville in August 1519 with five ships and about 280 men. Fourteen months later, after spending the severe (凌冽的) winter on the coast of Patagonia, he discovered the channel (海峡)

which is now called Magellan Straits. In November 1520, after many months of dangers from rocks and

storms, the three remaining ships entered the ocean on the other side of South America.

     They then continued, hoping to reach Asia. But they didn't see any land until they reached the islands

off the coast of Asia. Before they arrived at these islands, later known as Philippines, men were dying of

starvation. While they were in the Philippines., Magellan was killed in battle.

     The remaining officers then had to get back to Spain. They decided to sail around Africa. After many

difficulties, one ship with eighteen men sailed into Seville after leaving. They were all that remained of

Magellan's expedition (探险队).

1. Magellan Straits were discovered by Magellan ______.

A. in August 1519

B. in October 1520

C. in November 1520

D. in December 1520

2. The number of the ships lost on the whole expedition was ______.

A. two

B. three

C. four

D. five

3. Which of the following is NOT mentioned in the text?

A. Dangers from rocks and storms

B. Lack of fresh food and water

C. Severe winter in Patagonia

D. The death of Magellan