Definition: someone or something that you do not know and so you cannot be certain about Level: C2
那个地区的路况有点不确定。
The road conditions in that area are a bit of ___an ___unknown ___quantity.
如上所述，激光束甚至对其发明者来说都是一个未知数。
As mentioned above, the laser beam was ___an ___unknown ___quantity even to its inventor.
他对这个消息的反应仍然是个未知数。
His reaction to the news is still ___an ___unknown ___quantity.
