For many years, animals for carrying food and other supplies from one area to another has been a means of living in Xinjiang Autonomous Region .
A. trained
B. training
C. having trained
D. to have trained
网友回答
A. trained
(答案→)B. training
C. having trained
D. to have trained
解析：考查动名词作主语。本题的主语是动名词短语training animals for carrying food and other supplies from one area to another.谓语动词用单数形式。故B正确。C项是完成式，表示已经完成的动作，与句意不符。