The principal seagoing ship used throughout the Middle Ages was the galley, a long, low ship fitted with sails but driven primarily by oars. The largest galleys had as many as 50 oarsmen. Since they had relatively shallow hulls, they were unstable when driven by sail or when on rough water; hence they were unsuitable for the voyage to the East. Even if they hugged the African coastline, they had little chance of surviving a crossing of the Indian Ocean. Shortly after 1400, shipbuilders began developing a new type of vessel properly designed to operate in rough, open water: the caravel. It had a wider and deeper hull than the galley and hence could carry more cargo; increased stability made it possible to add multiple masts and sails. In the largest caravels, two main masts held large square sails that provided the bulk of the thrust driving the ship forward, while a smaller forward mast held a triangular-shaped sail, called a lateen sail, which could be moved into a variety of positions to maneuver the ship.

According to paragraph 3, what did the lateen sail contribute to the caravel as a sailing ship?

•A.It provided stability for the front part of the ship.

•B.It made it possible for the hull to be wider and deeper.

•C.It added considerably to the speed of the wind-driven ship.

•D.It improved the capacity of the ship to be guided.

显然，这是一道事实信息题，题目问lateen sail对caravel的功能是什么，直接可以定位到最后一句，可知lateen sail是一种三角帆，但对它的描绘就是最后的非限制性定语从句，可知它可以移动到不同位置控制帆船，对应D选项。所以这句话中非限制性定语从句就是解题关键。

接下来我们再看一道句子简化题：

The explanation is that the Maya excavated depressions, or modified natural depressions, and then plugged up leaks in the karst by plastering the bottoms of the depressions in order to create reservoirs, which collected rain from large plastered catchment basins and stored it for use in the dry season. 

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

•A.Southern Maya populations obtained the water they needed for the dry season by collecting and storing rainwater in sealed depressions.

•B.The Maya are credited with creating methods for modifying natural rainwater and storing it.

•C.Leaks in the karst caused difficulties in the creation of reservoirs, which were needed to store water for the dry season.

•D.Southern Mayans were more successful at collecting rain than storing it during dry seasons.

原句的essential information是玛雅人做了三个动作，即excavated depression、modified natural depressions 和plugged up……为了create reservoirs，其后便是非限制性定语从句对reservoir进行补充说明，也就是reservoir的作用，即收集雨水以便干旱季节使用的，因此这句话中非限制性定语从句也是至关重要的，不可忽视，表达出玛雅人是如何解决干旱季节的用水问题。