Experts say it is an international trend to __________ MP2.5 to check air quality.

Although many great people ever failed, theynever_______ and managed to succeed.

When he was told the ________ result of the research,there was a ________ expression appearing on his face.[ ]

If prices rise too high, the government has to do something ______ it.

It was last October ____ they finally came back totheir hometown.