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

Wang Ping hesitated for ___ before he ___ to the question.

All passengers _______, please fasten your seat belts and remain seated _______ the plane is landing.

—Do you often visit your friends?—________，because we are all busy with our jobs.

The mother looked at the door _______，expecting the children’s coming．