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1·We don't know for sure how much mass our Sun's lost over the last four billion years.

我们不知道我们的太阳在过去的40亿年里损失了多少质量。

2·Scientists are continually learning more about sleep, but one thing they know for sure is this—for humans, it is as important as food and water.

科学家正在不断加深对睡眠的研究，他们对其中的一件事实确信无疑：对于人类来说，睡眠与食物和水一样重要。

3·We'd probably know for sure whether someone was shooting at the president from that famous grassy knoll.

我们可能确定是否有人正在从那个著名的草地山丘草地上向总统射击。

4·Do you remember all years when scientists argued that smoking would kill us but the doubters insisted that we didn't know for sure?

还记得那些年吗？当初，科学家提出吸烟会导致死亡，而怀疑者们却坚持认为我们还不能确认这一点。

5·Just as on smoking, voices now come from many quarters insisting that the science about global warming is incomplete, that it's OK to keep pouring fumes into the air until we know for sure.

就像吸烟一样，现在来自许多方面的声音坚持认为，有关全球变暖的科学是不完整的，在我们确定之前，继续向空气中排放烟雾是可以的。

6·Well, I don’t know for sure either.

我也不知道。

7·However, we don't know for sure.

然而，我们仍然不能确信。

8·But Idon \ 't know for sure.

但我也不敢肯定。

9·We may never know for sure.

我们可能永远无法确定。

10·What do you know for sure?

您肯定的事情是什么？