By the end of this year Mr.Smith ________in our company for exactly three years.
问题描述：
By the end of this year Mr.Smith ________in our company for exactly three years.
为什么中间是填 will have worked
还可以填 has worked
答
By the end of this year截止到年底
因为还没有到年底,又强调一个完成的状态
要用将来完成时