The speed ______ Tom drives his car is too high.
The Australian boy has a good ________ of Chinese and can express himself freely in it.
—How can I use the equipment?—Well, just refer to the ________.
—Do you know why Jane was so angry?—Her roommate gave in to ________ and opened the letter addressed to her.
In my brother's absence, I would like to thank all concerned on his ________.
—What does PM 2.5 mean?—It's a professional ________ indicating the condition of air pollution.