They found that over a period of six years, planting three rows of trees reduced total dust and ammonia by more than half.

VOA: special.2009.09.08

The rock formations there are the result of deposits made by ancient lakes and rivers over a period of about twenty million years.

VOA: special.2009.04.15

Now there are so many,experts say up to twenty metric tons a day could be removed with nets ver a period of several years.

VOA: special.2009.12.15

Volunteers received vaccinations over a period of six months and were tested for H.I.V.

VOA: special.2009.09.30

"The Lost Symbol" takes place in Washington,D.C., over a period of just twelve hours.

VOA: special.2009.10.09

Instead,slow earthquakes release their energy over a period of hours or even days.

VOA: special.2009.07.07

It also can develop slowly, over a period of days.

VOA: special.2010.07.27

And over a period of time her grades fell completely.

VOA: special.2010.04.22

That is a period of growth followed by another downturn.

VOA: special.2009.08.21

Volunteers received vaccinations over a period of six months.

VOA: special.2009.10.13

Rome was going through a period of civil war.

VOA: special.2011.02.16

over a period of three years.

VOA: special.2009.07.17

"A team of doctors, including emergency physicians and cardiologists, attempted to resuscitate him for a period of more than one hour,"

VOA: standard.2009.06.26

"We've called for a total cessation of all new settlement building for a period of 10 months,"

VOA: standard.2009.11.26