President Pierce found it difficult to control them.

VOA: special.2009.04.16

And it was difficult to control.

VOA: special.2009.09.10

However,this is more difficult because it is harder to control the diets of people and because they live longer than other animals.

VOA: special.2009.09.01

Unless carbon emissions are brought under control,he says, the challenge of adapting to global climate change will become ever more difficult.

VOA: standard.2010.06.14