—Is there any vaccine(疫苗) against H1N1 flu that can be used on humans?
—No, but scientists are working to develop ________ .
| A．that | B．it | C．one | D．so |
23. Everyone should remember it is very important to _____ the balance of nature toady.
| A．get | B．make | C．keep | D．Remain |
22. She always ____ her opinion ____ facts.
| A．depends, on | B．bases, on | C．put, into | D．bases, by |
21. Every morning, the fish mongers stood there ______ with the fishermen for fishes.
| A．to argue | B．to talk about prices | C．bargaining | D．Discussing |
20. When Jack arrived, he learned Mary ____ for almost an hour.
| A．had gone | B．had set off | C．had left | D．had been away |
19. They didn't _____ the wedding because of a heavy rain.
| A．join | B．take part | C．attend | D．Appear |