There isn't so much pollution in the coiuntry in big citi

題目：

There isn't so much pollution in the coiuntry () in big cities
that/as/which/what
是as麼?是因爲前面的so,還是因爲比較時用as

解答：

as
不是因爲so,而是因爲後面不是完整的句子.不是定語或主語從句that/which/what都不對.
as in big cities,與前面比較,是簡單句成分.