HSK 7-9
难以置信
nányǐ-zhìxìn(idiom) hard to believe; incredible
眼前的变化快得难以置信。
Yǎnqián de biànhuà kuàide nányǐzhìxìn.
The change before us happened unbelievably fast.
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Appears across levels (7-9) in the HSK 3.0 word list.