Which of the following is an example of biological weathering?
a. plant roots growing through rock
b. water freezing in the cracks of rocks
c. oxygen reacting with the minerals in rock
d. rocks breaking from thermal expansion
frost wedging weathers rock because
water expands when it freezes
oxygen expands when it is heated
water contracts when it freezes
plant roots create external pressure

Biological weathering is caused by living organisms, such as microbes, animals, and plants. Water, oxygen, and rocks are nonliving things (though water and rocks may be capable of weathering), and only plant roots fits the definition of biological weathering. So the correct answer for #1 is A.

Frost wedging occurs when the little water content in rock freezes, expanding and forcing the rock apart. Eventually, enough freeze-melt cycles causes the rock to break apart little by little. This is due to ice's density being less than that of water, meaning that it is because water expands when frozen. So the answer to #2 is A.