故选mountains。

　　15．A.by and by B．here and there

　　C．over and over D．up and down

　　解析：选C　句意：达·芬奇喜欢研究岩石，因此，在他的其他绘画作品中可以多次看到石头的影子。by and by"不久以后"；here and there"到处"；over and over"多次；反复"；up and down"上上下下"。

　　16．A.on B．by

　　C．to D．beyond

　　解析：选C　句意：那位妇女双腿侧向一边坐着。

　　17．A.caught B．held

　　C．supported D．hung

　　解析：选B　句意：她的双手叠放在前面。hold one's hands"握着手"。

　　18．A.painting B．living

　　C．smiling D．sitting

　　解析：选D　句意：这种坐姿现在也被许多模特采用......根据上一句可推知此处应选sitting。

　　19．A.women B．actresses

　　C．girls D．models

　　解析：选D　models此处意为"模特们"。

　　20．A.painted B．picked

　　C．photographed D．chosen

　　解析：选A　句意：......当模特们被画的时候。由上文可知，此处应选painted。

　　Ⅱ.短文改错

　　During this winter vacation, I work as a volunteer guide in the National Museum of China. It was a extremely unforgettable experience. A few day before the vacation, I applied for the job online. Shortly afterwards, I was lucky enough to be give the job. Then I took a 5­day training course before the vacation began. I was taught what to be both an informative and entertaining tour guide. It was very usefully, as I had not done this type of work before them. Over the followed two weeks, I worked as a guide around the Silk Road exhibition, that gave me a useful insight into the history. At end of the vacation, I was delighted to be praised for being an outstanding volunteer.

答案：第一句：work→worked