What does the word “it” in paragraph 4 refer to?

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 44 to 48.

Sylvia Earle is an underwater explorer and marine biologist, who was born in the USA in 1935. She became interested in the world's oceans from an early age. As a child, she liked to stand on the beach for hours and look at the sea, wondering that it must be like under the surface. When she was 16, she finally got a chance to make her first dive. It was this dive that inspired her to become an underwater explorer. Since then, she has spent more than 6,500 hours under water, and has led more than seventy expeditions worldwide. She has also made the deepest dive ever, reaching a record-breaking depth of 381 meters.

In 1970, she became famous around the world when she became the captain of the first all-female team to live under water. The team spent two weeks in an underwater "house". The research they carried out showed the damage that pollution was causing to marine life, and specially to coral reefs. Since then she has written several books and magazine articles in which she suggests ways of reducing the damage that is being done to the world's oceans. One way, she believes, is to rely on fish farms for seafood, and reduce the amount of fishing that is done the out at sea.

(Adapted from Complete IELTS by Rawdon Wyatt)

By then, the elite performers had all totaled 10000 hours of practice over the (43) ________of their lives, the merely good students 8000 hours and the future music teachers just over 4000 hours.