Read the following passage and circle the letter A, B, C, or D to indicate the correct answer to each of the questions.

Leonardo da Vinci was one of the greatest painters, architects, and thinkers in his era. His works have influenced not only other artists and painters, but also people working in scientific fields.

He was born in 1452 in a small town near Vinci, Italy, where his surname comes from. Leonardo loved nature and spent a lot of time drawing pictures of animals and plants. He received little formal education, but his uncle and father soon discovered his talent, so they decided to send him to a workshop in Florence to apprentice with the master artist Verrocchio. Da Vinci not only improved his skills in drawing, painting, and sculpting, but he also picked up knowledge and technical skills in other areas including mechanics, carpentry, metalwork, and architecture.

In 1472, Leonardo qualified as a master artist and opened his own workshop. However, he still continued to work together with Verrocchio for the next five years. After that, he started working independently and completed a painting for the Duke of Milan. His work made a huge impression on the Duke who decided to offer him a position in his court. While in Milan, Da Vinci was invited to design innovative buildings, machines, and weapons. He would draw up plans for several inventions of the future such as tanks, submarines, and flying machines. He was also skilled at drawing detailed sketches of the human body, He left behind several notebooks filled with diagrams, drawings, observations, and notes of his studies, ideas, and inventions.

Unfortunately, his interests were so broad that he often changed his focus to a new project and rarely finished the works he had started. The Last Supper and Mona Lisa were two of the masterpieces he could finish during his lifetime. In 1495, he started The Last Supper, which took him nearly three years to complete. In 1503, he began painting the Mona Lisa and worked on it from time to time over several years until his death.

Da Vinci died in 1519, at the age of 67, but he is still considered as one of the greatest geniuses of all time. His paintings changed the world of art, and many inventions were carried out based on his ideas and sketches. His greatness lies in the depth and diversity of his knowledge, and 500 years after his death his genius shines as bright as ever.

The word they' in paragraph 2 refers to ________. A. Da Vinci's teachers B. Verrocchio's apprentices C. Da Vinci's uncle and father D. Da Vinci's sculptors