实际上就是：
Predominantly by the process (of urbanization), towns and cities are formed and become larger as more people begin living and working in central areas.

或者

Towns and cities are, predominantly by the process (of urbanization), formed and become larger as more people begin living and working in central areas.

【 在 McJohn 的大作中提到: 】
: It is predominantly the process by which towns and cities are formed and become larger as more people begin living and working in central areas.
: 这个by是对process的解释，表示通过以下的原因导致……
: which代表的是process，它是这个长句后半部分(by之后)的主语，从towns到句尾都是在阐述which所代表的process.
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FROM 115.27.206.*