—What does PM 2.5 mean?—It's a professional ________ indicating the condition of air pollution.
---- “How did the robber get in?” ----“______ an open window on the first floor.”
_______ reading the letter, what has he done?
_____ their inexperience, they’ve done a good job.
----“What a terrible rain we are having!” ----“Yes. We are asked to pay more attention to the information about the rain ______ flood.”
In the west, people make _______ a rule to send Christmas present to their relatives and friends.