Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 38 to 42.

(1) Though Edmund Halley was most famous because of his achievements as an astronomer, he was a scientist of diverse interests and great skill. In addition to studying the skies, Halley was also deeply interested in exploring the unknown depths of the oceans. One of his lesser-known accomplishments that were quite remarkable was his design for a diving bell that facilitated exploration of the watery depths.

(2) The diving bell that Halley designed had a major advantage over the diving bells that were in use prior to his. Earlier diving bells could only make use of the air contained within the bell itself, so divers had to surface when the air inside the bell ran low. Halley’s bell was an improvement in that its design allowed for an additional supply of fresh air that enabled a crew of divers to remain underwater for several hours.

(3) The diving contraption that Halley designed was in the shape of a bell that measured three feet across the top and five feet across the bottom and could hold several divers comfortably; it was open at the bottom so that divers could swim in and out at will. The bell was built of wood, which was first heavily tarred to make it water repellent and was then covered with a half-ton sheet of lead to make the bell heavy enough to sink in water. The bell shape held air inside for the divers to breathe as the bell sank to the bottom.

(4) The air inside the bell was not the only source of air for the divers to breathe, and it was this improvement that made Halley’s bell superior to its predecessors. In addition to the air already in the bell, air was also supplied to the divers from a lead barrel that was lowered to the ocean floor close to the bell itself. Air flowed through a leather pipe from the lead barrel on the ocean floor to the bell. The diver could breathe the air from a position inside the bell, or he could move around outside the bell wearing a diving suit that consisted of a lead bell-shaped helmet with a glass viewing window and a leather body suit, with a leather pipe carrying fresh air from the diving bell to the helmet. 

The subject of the preceding passage was most likely Halley’s _____.