【正确答案】 1、they could speak the native language    

【答案解析】本题的关键词是The important advantage the Andean teachers。文中第十一段第三句"Though they had not had much training they had the help of great interest and, most important, they knew the native language." 即为 答案的出处，因此本题空格处应填they could speak the native language。