请问下面句子中，part of what defined them as
female 为什么是前面it的同位语，what 可以换成 which引导非限定从句吗？另外，define后面的them指girls还是指前面的children？如果指girls怎么理解呢？
It was not until the mid-1980s,
when amplifying age and sex differences became a dominant children’s marketing
strategy, that pink fully came into its own, when it began to seem inherently
attractive to girls, part of what defined them as female, at least for the
first few critical years.
