How does the strontium help us know where the log in Chaco canyon came from

Chuska and San Mateo mountains.

Explanation:

The strontium isotopes indicate that timbers and logs used to build houses and infrastructures at Chaco Canyon came from Chuska and San Mateo mountains.

These mountains are about 100 kilometres away from the canyon and Strontium ratio from the Chaco canon's dust, water and bedrock reveals that these construction beams were from the Chuska and San Mateo mountains.

the answer would be:

b. race