Air pollution is getting                            serious in big cities in the world.

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 26 to 30.

Those who work at home may find that the solitude can be a double-edged sword. It is, of course, easier to (26) ______ when you're in your own home with no co-workers coming by your desk to chat at random times. But while this solitude can feel blissful at times, when we have no mandate for social interaction during the workday—when we don't automatically run across people outside of those we live with—we can become lonely before we realize it.

Social media can feel like a lifeline to others, but this type of (27) ______ can sometimes feel isolating as well, as these interactions can feel less personal than face-to-face encounters and conversations. (28) ______ we may not need to resort to painting a face on a volleyball and talking to it, the feelings of isolation can sneak up and we can feel (29) ______ alone than we expect. And given the research on loneliness, this doesn't always feel good. Social isolation was a factor mentioned in the UN study (30) ______ was related to increased stress of home-based workers.