It __is_________(be) three years since Lily joined the Green.为什么不是 has been

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It __is_________(be) three years since Lily joined the Green.
为什么不是 has been

it is ..since 结构意为“从...起已经多长时间了” 可与 it has been ..since替换

基本上没有区别,可以用has been。