my mother ( )work on a farm has to,have to,has,have填哪个

题目

my mother ( )work on a farm has to,have to,has,have填哪个

答案

应该是
has to
我妈妈不得不在农场工作.
主语三单,一般现在时,所以必须用has
如果用完成时,后面动词work为过去分词才行,所以只能是has to
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