The old couple often take a walk after supper in the park with their pet dog ___.
20. The flu is believed________by viruses.
19. They________a lot of suggestions for improving the workers’ living conditions.
18. After having gone____-far, I didn’t want to turn back.
17. One of the mottos for the park makes_____clear that science and business can and must work together to build the future.
16. This experiment turned out to be____failure, but, as we know, success often comes after___failure.