PTE考生目前最大的问题之一就是练习题缺乏。除了有限的基本官方书（PLUS，Testbuilder, OG）之外

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60秒科学节目（SSS）是科学美国人网站的一套广播栏目，节目内容以科学报道为主，节目仅一分钟的时间，主要对当今的科学技术新发展作以简明、通俗的介绍，对于科学的发展如何影响人们的生活环境、健康状况及科学技术，提供了大量简明易懂的阐释。

The largest animal on Earth is thriving anew, at least off of California. And all it took was for people to stop hunting blue whales.That’s according to a new study in Marine Mammal Science.

Today, the California group of the mighty blue whales numbers roughly 2,200, according to conservationists and based on distinctive singing. That compares with the more than 3,000 killed over the first seven decades of the 20th century. The blue whale can grow up to 30 meters in length and weigh more than 180 metric tons—and requires for food each day roughly 40 million tiny sea creatures called krill.

This may explain why the number of whales off California has stopped growing. Some conservationists had argued that individuals being hit and killed by ship traffic held back population growth. But the new research suggests it’s more likely that California blue whales have reached the limits to growth in that region of the eastern Pacific Ocean.

Of course, more than 350,000 blue whales were killed in other parts of the world in the 20th century. So the species has hardly recovered to its historical abundance. But, at least in California, the blue whale is back.

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