they were pushed farther and farther until they were confine

。what 再问： why? 再答： b。 what 你可以用排除法来分析这个句子就会比较容易懂。 a。 。。。 they were confined to a reservation in (which) is now South Dakota。 这个which 从句的语序不对。后面要改成 South Dakota is now xxx。比方说 a reservation in (which) southerners live now。 c。 。。。 they were confined to a reservation in (where) is now South Dakota。 这个句子有两个毛病。 首先where 做关系副词时，前面不加介词。还有就是语序不对。 应该是 they were confined to a reservation where South Dakota is now。 What 在从句中可以当主语，they were confined to a reservation in (what) is now South Dakota。 后面的语序不变。