听力原文： Born in 1879, in Ulm, Germany, Einstein was two years old when his parents moved to Munich. There his father opened a business in electrical supplies. As a boy, Einstein did not learn to talk until later than others of his age, and in his early childhood he was not considered especially bright. But by the time he was 14 years old, he had taught himself advanced mathematics from textbooks. By them he knew what he wanted to be when he grew up. He wanted to be a physicist and &vote himself to research.
Einstein could not afford to pay for the advanced education he needed, because his family business had declined. Later, he and his family were forced to leave Munich to live in Milan, Italy, where they had relatives. As for him, the family did manage to send him to a technical school in Switzerland, and later to the Federal Institute of Technology in Zurich. In 1901, when Einstein was 22 years old, he began teaching, and in 1902, be went to work as a patent office examiner in Bern. Now able to pay his own expenses, he continued his schooling at the University of Zurich, where he received a doctor's degree in 1905. This was the period when he first began the research, which led to his famous theory of relativity.
Toward the end of his life, when Einstein was asked to explain his law of relativity to a group of young students, he said, "When you sit with a nice girl for two hours, you think it's only a minute. But when you sit on a hot stove for a minute, you think it's two hours. That is relativity."
What is Einstein's greatest contribution to human beings？

A. His teaching.
B. His theory of relativity.
C. His theory on advanced mathematics.
D. His research.