II.

　　3. 1) There was an unusual quietness in the air, except for the sound of artillery in the distance.

　　2) The expansion of urban areas in some African countries has been causing a significant fall in living standards and an increase in social problems

　　3) The research shows that atmospheric carbon dioxide levels are closely correlated with

　　global temperatures

　　4) The frequency of the bus service has been improved from 15 to 12 minute recently.

　　5) The diver stood on the edge of the diving board, poised to jump at the signal from the coach.

　　4. Automobiles have, since the ..........