However, certain challenges may not require the models at all.

This is where bonuses are given if you accomplish a certain challenge.

Consider whether you will act as an individual business or take a more co-ordinated approach to address certain environmental challenges.

But while they stand by, high-profile wives present certain challenges.

The California regulation still faces an almost certain legal challenge from the auto industry.

However, since some of the dances are very simple, there were certain challenges each team had to do while performing.

"But on the other hand, its uniqueness offers a certain challenge."

I can see that we will have to confront certain challenges in order to come up with effective regulations.

The countries of Europe are faced with certain challenges that they cannot resolve on their own.

"They require certain challenges in order to remain alive and vital."