Do you have any difficulty ________?
--The research on the new bird flu virus vaccine is challenging and demanding. Who do you think can do the job?--________ my students have a try?
--How much salt did you put in the soup?--I’m sorry to say, ________. I forget.
________ the important documents were needed_______.
We are waiting until the weather ________ a little.
--He must have received a good education in his childhood, ________ he?--________, he is polite and well-behaved and acts like a gentleman.