其实这道题吧,是因为if后面省略了主语和谓语.
写全了的话是这样的：The sound of planes is likely to cause deafness if (it is) heard continually.
如果选A的话,你读一下,The sound of planes is likely to cause deafness being heard continually.就没有了假设的韵味了.
if条件句如果从句和主句的主语和谓语是一样的话,从句可以省略主语和谓语.这种语法现象还是挺常见的,要掌握住哦!
有什么问题可以继续问~
加油!