1.   Can Thompson make a credible challenge for the party leadership?

2.   Chess posed a formidable challenge for computer scientists.

3.   Climbing Everest is a daunting challenge for any mountaineer.

4.   Detecting secret compartments remains a challenge for immigration inspectors, Ward said.

5.   How to deal with waste is a daunting challenge for the west.

6.   In another way too, the advance of science has posed a challenge for theology.

7.   Increased leisure time is a challenge for older people and most will welcome the challenge.

8.   It is a great challenge for the West.

9.   McConnell beat off a challenge for his Senate seat.