While he was investigating ways to improve the telescope, Newton made ____ discovery which completely changed ________ man’s understanding of color.
We were swimming in the lake _____ suddenly the storm started.
---Have you read the timetable ? ---Yes . The train ____ at 10:12 p.m..
Do you know __________ ?
The message is very important, so it is supposed _______ as soon as possible.
Visiting _______ Great Wall is ________ unforgettable experience to me.