although some quite difficult
虽然有些相当困难
以上为机器翻译结果，长、整句建议使用 人工翻译 。
Although some sightings of UFOs are difficult to explain, most can be explained quite easily.
虽然有些目击 UFO 的现象很难解释，但大多数都可以很容易地解释。
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