高一名词性从句3With Miss Sullivan's patience and tenderness,little

高一名词性从句3
With Miss Sullivan's patience and tenderness,little Helen ___ people all thought would not have lived for long overcame her deafness and blindness and became a good writer.
A.who B.whom
为什么选A?还原句子是：People all thought little Helen would not have lived for long overcame her deafness and blindness and became a good writer.为什么在动词后面不用宾格?