With more and more farmers moving to cities, an inadequate labor force is    becoming a major factor affecting the country's grain output.

Tell your brother to come _____, because it's going to rain in a minute or two.

The questionnaire takes        ten to fifteen minutes to complete and can be used long with the assessment interview.

He tried his best to solve the problem, _____ difficult it was.

There is still much to discuss. We shall, ____, return to the problem next time.