Please answer the following question: Read the article and select the best answer. Article: When we talk about village teachers, we often think of difficult conditions, heavy teaching tasks and little income. But there is a village teacher who makes his teaching jobso interesting. His name is Liu Yuesheng who is now called "the most beautiful teacher", Liu comes from Liugang Village Middle School in Dagang, Tianjin. The school is a rural area middle school. It has only about more than 300 students and teachers. Though the school is small, it is only a village school that takes the municipal scientific research projects in Tianjin. Since Liu started to work at Liugang Village Middle School in 1999, he has led his students completing the 300 designs and reportings of national patents . Liu spent three years writing a book called Practice and Invention. In this book, he mostly tells people his teaching experience. The inventions of his students won the silver medal in the national youth science and technology competition. The rural students improve their living skills while they are inventing. Many of the students start their own business after graduation . Liu also guided his students to make a big 260 Model Airplane. He often takes his students to go to the wetlands and take photos during holidays. In order to call on people to pay more attention to protecting environment, he further watches the number of the birds with his students. Today , the wetland aerial of his school of has reached the UAV level. Question: Liu yuesheng has been teaching Liugang Village Middle School for _ years. Options: A: 20 B: 18 C: 15 D: 12
A:
C