[求助]这道语法题法家给讲讲

可能这两天一直在看语法，脑子有点混乱了，这道题怎么也想不明白了，大家给讲讲

During the 18th century, Little Turtle was chief of the Miami tribe whoes territory became __B___is now Indiana and Ohio.

A. where. B what

为什么不是where?

what is now

gu ding yong fa

hehe,i cannot type chinese so just Ping Yin.

Because  "where" is not used as a subject,  "what" can be. "where" is only used as a adverbial modifier.