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【往期回顾】
今天我们接着来看第91课的“每课一句”。
一、课文原文
Lesson 91 Three men in a basket
A pilot noticed a balloon which seemed to be making for a Royal Air Force Station nearby. He informed the station at once, but no one there was able to explain the mystery. The officer in the control tower was very angry when he heard the news, because balloons can be a great danger to aircraft. He said that someone might be spying on the station and the pilot was ordered to keep track of the strange object. The pilot managed to circle the balloon for some time. He could make out three men in a basket under it and one of them was holding a pair of binoculars. When the balloon was over the station, the pilot saw one of the men taking photographs. Soon afterwards, the balloon began to descend and it landed near an airfield. The police were called in, but they could not arrest anyone, for the basket contained two Members of Parliament and the Commanding Officer of the station! As the Commanding Officer explained later, one half of the station did not know what the other half was doing!
——选自《新概念英语》第二册，外语教学与研究出版社，1997年10月第1版
二、每课一句
The officer in the control tower was very angry when he heard the news, because balloons can be a great danger to aircraft.
注意本句话的结构：
1、第一层关系是由because引导的原因状语从句，用来解释控制塔上的军官之所以生气的原因；
2、第二层关系是在前面的主语中出现的由when引导的时间状语从句，用来说明控制塔上的军官是什么时候发怒的。
本句话更加需要大家注意的是because引导的从句谓语动词采用了can be的形式，即一般现在时的形式，而其主句的谓语动词却采用的是was very angry，即一般过去时的形式。
之所以这样，是因为because从句所给出的原因是一个客观事实，即无论何时对于飞机来说，气球都是一个很大的威胁。既然是客观事实，就需要用一般现在时来描述。
此外，要注意danger在这句话中的用法，即：
sth be a danger to sth.
...对...来说是个威胁
如：
A careless driver is a danger to us all.
粗心的司机对我们大家都是一种危险。
三、更多拓展
He informed the station at once, but no one there was able to explain the mystery.
注意这句话中的there意指“基地”，用作后置定语，修饰no one。
此外，要注意本句话中的was able to表示“具有...能力；得以...”，在此不用could，是为了凸显气球的出现是个较大的谜团，大家都无法解释是怎么回事。
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