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How did scientists determine the origins of the ancient dogs found in Alaska?
A．By analyzing the age of their bones.
B．By examining the animals' DNA.
C．By studying the shape of their skulls.
D．By comparing them with modern dogs.
解析：选B 细节理解题。根据第三段尾句中的“another DNA study has already shown that ancient dogs preserved in the Alaskan ice fields evolved from Asian wolves, not American ones”可知，科学家是通过检测DNA的方法来判断阿拉斯加狗的起源的，故B项正确。