Read the following passage carefully and choose the most appropriate answer to the question out of the four alternatives.
Marie Curie was one of the most accomplished scientists in history. Together with her husband, Pierre, she discovered radium, an element widely used for treating cancer, and studied uranium and other radioactive substances. Pierre and Marie's amicable collaboration later helped to unlock the secrects of the atom.
Marie was born in 1867 in Warsaw, Poland, where her father was a professor of physics. At an early age, she displayed a brilliant mind and a casual personality. Her great exuberance for learning prompted her to continue with her studies after high school. She became disgruntled, however, when she learned that the university in Warsaw was closed to women. Determined to receive a higher education, she defiantly left Poland in 1891 entered the Sorbonne, a French university, where she earned her master's degree and doctorate in physics.
Marie Curie's doctorate was in which academic discipline?