challenging

The test was designed to operate seamlessly in Florida's varied and challenging environments, from dense Everglades habitats to urban areas where non-native constrictors are now found.

The filmmaker found her on a visit to her Essex, England, school, looking for the perfect kid for her coming-of-age drama about a young girl growing up in challenging circumstances.

The change in Sacramento was fueled in part by major turnover in the Legislature this year, creating new opportunities for candidates to run without challenging an incumbent.

Obviously I hope they don't get back to what they were under Sir Alex Ferguson but wouldn’t it be great to have four or five clubs challenging for the title?

Electrospinning using a starch-rich ingredient such as white flour is more challenging than using pure starch, as the impurities -- the protein and cellulose -- make the mixture more viscous and unable to form fibres.