A. To fly through the water looking for explorers.
B. To show TV programs and films to experimental workers at sea.
C. To demonstrate technology for the development of another vehicle of its kind.
D. To transport explorers to the very bottom of the sea.

A. Birds use their beaks to clean their feathers.
B. Preening is very important to birds.
C. The habits of birds are interesting.
D. How birds clean their feathers.