1.
it was a series of separate walls
“extending” across northern China.
想问为什么不用被动extended
2.
I'll finish it later “on”.
请问on是惯用语还是有其他说明跟解释
谢谢大家

1. 墙本身一直延展出去，不是被某人延展。2. later on跟later基本上可互换。