S e c o n d e d I t I o n 1 Reading for the Real World 1

4.
An __________ is a community of living and non-living things that work 
together.
5.
It is important to __________ water when living in the desert.
6.
It is far easier to see __________ in rural areas than in the city.
Fill in the blanks with the phrases from the list. Use each phrase only once.
Human activity is a primary reason why 
1 
_________________ are now 
extinct and many more are endangered. The problem is that habitats are 
2 
_________________ not by one single event, but by a series of actions. 
One example is when people 
3 
_________________ or animals to a new 
environment where they have no natural predators. This risks disrupting the 
ecological balance in the area and 
4 
_________________. Humans also damage 
the Earth by 
5 
_________________ for development and agricultural purposes. 
If this trend 
6 
_________________, the future of the planet will be a grim one 
indeed.
being destroyed 
introduce alien plants 
kills native species
continues unchecked 
numerous species 
cutting down forests

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ixty-five million years later, the extinction of the dinosaurs remains a 
great mystery. Scientists think that dinosaurs existed on Earth for 
almost 200 million years. How could these great beasts, some of which 
weighed thousands of pounds and stood 100 feet tall, suddenly disappear?
The most popular theory is that dinosaurs were killed off when an asteroid 
crashed into southern Mexico. The asteroid’s collision caused earthquakes, fires, 
and tidal waves. Volcanoes erupted, spewing poisonous gases into the sky and 
lowering the oxygen level in the oceans. Plants died, removing the food source 
for plant-eating dinosaurs. As these dinosaurs died, there was no food for 
meat-eating dinosaurs. In a short period of time, the dinosaurs were gone, and 
the first mammals began to appear.
Many scientists note that, while the asteroid had a major impact, the Earth’s 
climate had already begun to change. The planet was cooling, and the colder 
temperatures were likely killing plants. Some scientists claim that mammals already 
on Earth before the asteroid might have facilitated the 
extinction by eating dinosaur eggs. 
We may never know for certain what caused the 
extinction of dinosaurs. But it was most likely the result 
of a combination of the asteroid, colder climates, disease, 
and egg-stealing mammals rather than the single event of 
the asteroid hitting the Earth.