Scientists say it may be five or six years ____ it is possible to test this medicine on human patients.

She is too busy to help us finish the work.— Let’s do it ________.

Emma ，can you introduce _______ to Alice ?I want to meet her .

The population of the world has grown very fast ____ four hundred years. [ ]

When did the Shanghai Expo open?—_____ May lst, 2010.[ ]