The sky was a mass of black cloud out of _____ lightnings flashed.
请问如何填?
答案填 which。句意如下:
The sky was a mass of black cloud out of which lightnings flashed.
其中的关系代词 which 指代 a mass of black cloud。
句意:天空乌云密布,闪电从中掠过。
原句相当于:
The sky was a mass of black cloud and lightnings flashed out of it.