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难句解析：
①"But", he cautions, "some people may have the idea that once one understands the causes, the cure will rapidly follow. Consider Pasteur, he discovered the causes of many kinds of infections, but it was fifty or sixty years before cures were available."
▲此句是一个直接引语，注意引语中有两个句子，前面的句子有一个复杂的同位语从句that once one understands the causes, the cure will rapidly follow修饰the idea，而后面提到了一个例子，句中有一个but，它后面的it指代前面整句话。
△注意引语中两个句子之间的关系，后者是用来驳斥前面一句中那些人的错误想法的，作者在这里举的是一个反例。
②The researchers made great progress in the early 1970s, when they discovered that oncogenes, which are cancer-causing genes, are inactive in normal cells.
▲此句主句是The researchers made great progress in the early 1970s，后面有一个when引导的定语从句修饰the early 1970s，而在这个定语从句中还有一个宾语从句，在宾语从句中又有一个非限定性定语从句which are cancer-causing genes修饰oncogenes。
△注意此句中有超纲词oncogenes，但是我们可以从后面修饰它的定语从句推出其确切含义，这也是常用到的阅读方法之一，即可以用定语从句来反推所修饰词的意思。
③The exact mechanisms involved are still mysterious, but the likelihood that many cancers are initiated at the level of genes suggests that we will never prevent all cancers.
▲此句是用but连接的复合句，注意前面的involved表示“相关的”，而后面的半句的主干是the likelihood suggests that...，里面又包含有一个同位语从句和宾语从句。
△注意看清句子结构，而后面的likelihood表示“可能性”。
④Environmental factors can never be totally eliminated; as Hayward points out, "We can't prepare a medicine against cosmic rays."
▲此句用分号隔开，前面是一个简单的被动语态，而后面是一个直接引语。
△如果我们了解分号的用法，就知道它经常用来连接两个意思相仿或相对的句子。而在这里，分号后面的部分就是进一步说明前面的意思，那么我们也就知道cosmic rays是environmental factors的一个个例。
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