But the main problem ---------- deeper and will take longer

un deep 是固定短语, 意为“根深蒂固”“植根很深”“深藏不露”
如：

She's never shown any sign of having a mind of her own.Sill waters run deep.
她从不流露自己有什么想法.静水流深啊.
And sometimes our differences run so deep; it seems we share a continent, but not a country.
有时,我们的分歧是如此之深,似乎我们虽身处同一个大陆,但不属于同一个国家.


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