A rock climber hangs from a ledge while others look on .
We doubt that a rock climber hangs from a ledge while others look on because he might fall .
We doubt that a rock climber hangs from a ledge while others look on because he might fall .
It is probable that a rock climber hangs from a ledge while others look on because he might fall .
A rock climber hangs from a ledge while others look on because he might fall .