Frankie is watching a documentary on the history of diving. What is he MOST likely to hear about as the first method of underwater exploration?
A. a scuba, which is a self-contained underwater breathing apparatus
B. diving bells, which are self-contained underwater breathing apparatuses
C. a scuba, which is a chamber that can be lowered to the sea floor
D. diving bells, which were chambers that can be lowered to the sea floor