44. _____ the _____ of modern industry, the pollution is becoming more and more serious.
——Tomorrow ____ my birthday. I’d like you and Jane to come. ——I’m not sure if she ____ free.
This problem may lead to more serious ones if ____ unsolved.
She felt sorry that she had disturbed the ____ calmness of a man she respected.
If you live in the country or have ever visited there, ____ are that you have heard birds singing to welcome the new day.
His letter was especially welcome as I ____ from him for long.