核心问题
At first, the more dense, dark, copper sulfate solution is in the bottom half with pure water floating on top of it. [北大《英语》(三), p. 57]
[原译]首先，把较浓黑的硫酸铜溶液装满半筒，然后在上面装满纯水。（某教参）
at first 可以译为“首先”吗？
词典解答
应当记住, at first 与 first 在词义上是不同的。first 可作“首先”“第一”解, 但 at first 则作“最初”解。例如:
-
He found English difficult at first, but soon mastered it. 起初他认为英语是很难学的，但是他很快便掌握了。
-
At first people had refused to believe such a thing possible. 起初人们并不认为这样的事情是可能的。
[试译]起初,密度较大的深色的硫酸铜溶液位于量筒的下半部,清水浮在上面。请做下面的英语试题:
____ I didn't understand the job but now I'm making progress. A. On the beginning B. At first C. For a start D. In principle(Nelson,400C)