Miami will be the first and perhaps most difficult challenge.

To prove herself, she took on the company's most difficult challenges.

We've always talked about this being the most difficult challenge of all.

Sharing with these other children may be the most difficult challenge of all.

Even families who have the time and resources face a difficult challenge.

"If the amount changes every year, that is the single most difficult challenge I face as a state college president."

In his many years, he had never faced a more difficult challenge.

Your vision is needed as we face the difficult challenges ahead.

The fight against cancer is a difficult challenge, but one to which Europe and the world must rise.

Will we be capable of taking up this difficult challenge? I do not know.