Directions: In the following passage, some words have been deleted. Select the most appropriate option to fill in each blank.

Dolphins are regarded as the friendliest creatures in the sea and stories of them helping ___(1)___ sailors have been common since Roman times. The more we learn about dolphins, the more we realize that their society is more ___(2)___ than people previously imagined. They look after other dolphins when they are ill, care for pregnant mothers, and ___(3)___ the weakest in the community, as we do. Some scientists have suggested that dolphins have a language but it is much more probable that they ___(4)___ with each other without needing words. Certainly, the most common argument in favor of man's ___(5)___ over them that we can kill them more easily than they can kill us is the least satisfactory.