Humpbacks also appear to make social sounds—vocalization distinct from singing that appear related to male aggression and competition.
定语从句that appear related to male aggression and competition修饰谁?
appear为复数谓语动词,这就告诉你that指代的是复数名词,所以你结合逻辑意义,就能很容易的判断that的先行词为social sounds. 你平时学的语法知识要用在实际阅读中,与逻辑分析结合使用。
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