Space travel remains a difficult and expensive activity.

You will be required to be seriously fit for this trip as difficult activities are included.

Fatigue that makes normal activities difficult or impossible may linger for 2 to 3 months.

On the wooded campus, the men pooled their resources to complete difficult activities.

The calm nature of the place helps to forget from the difficult activities of daily life.

As the illness progressed, composition became an increasingly difficult activity.

The fur trade, often practiced without authorization, was a difficult activity, carried on most of the time by young unmarried men.

Getting your debtors to pay their bills can be a difficult and time-consuming activity.

A balance between ability level and challenge: the activity is neither too easy nor too difficult.

We need long-term stability for the difficult, labour-intensive and costly activity of milk production.