—Why do you look sad?—There are so many problems ________.
I went to bed early _____ the sleep lost last night.
After the bell rings, which indicates the ending of the exam, you_____ stay ____ you are until all your papers are collected.
Doing morning exercises _____ our health and we _____it.
The plan _____ by the Chinese government ____ rejuvenating the nation by relying on science.
---I apologized for_____ to tell you when and where the meeting will be held in advance.