Once new difficulties _________ in your work, it’s always important to use your brains.
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.