10. By the middle of the 21st century , the vast majority of the world’s population ________ in cities rather than in the country.
Though we don't know what was discussed, yet we can feel the topic __________.
—What is the price of petrol these days? —Oh, it __________ sharply since last month.
—Why does the lake smell terrible? —Because large quantities of water __________.
Progress __________ so far very good and we are sure that the work will be finished on time.
For many years, people __________ electric cars. However, making them has been more difficult than predicted.