People on the grassland of is now Kazakstan were the first to achieve both riding and milking horses．
A. where
B. which
C. what
D. as
网友回答
A. where
B. which
(答案→)C. what
D. as
解析：句意：现在所谓的哈萨克斯坦草原上的人们是第一个既实现了骑马也实现了挤马奶的名族。what在句中作为介词后接宾语从句，在句中作为主语，充当句子的成分，故选C