The word its in paragraph 4 refers to improvement Halley's bell source of air

Câu hỏi :

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. 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 exploring the unknown depths of the oceans. One of his lesser-known accomplishments that was quite remarkable was his design for a diving bell that facilitated exploration of the watery depths.