练习步骤：
step1：划分句子结构，找句子主干
step2: 确定修饰词，修饰短语或修饰从句的修饰关系
step3: 确定代词指代对象
剑桥雅思9
Reading Passage 2
长难句：An alien civilization could choose many different ways of sending information across the galaxy, but many of these either require too much energy, or else are severely attenuated while traversing the vast distances across the galaxy.