--Why are you so late ?--I was on the way when it suddenly__to me that I had left my notebook home, so I had to fetch it.
____ we is known to all, English is not very difficult to learn.
The old man had three sons, all of ____ died during World War Ⅱ.
I have bought two pens, ____ write well.
Do you know the reason ____ she has changed her mind?
He failed in the exam, ____ proves that he wasn’t working hard enough.