The exact causes of pms are not fully understood .

经前综合症的确切病因尚未完全了解。

But the technology is well understood .

但人们对核技术了解深刻。

I think he understood about plums .

我想他很了解李子。

The means by which aspirin prevents cancer is not well understood .

阿司匹林是如何预防癌症的还不为人了解。

However , these remained the least well understood asset classes .

不过，这些仍然是人们了解最少的资产类别。