When we use stratigraphy to classify sediments, we use a. the orientation of the magnetic field at the time it was deposited b. all of these things and anything else we can find to establish a sequence of formation c. the nature (properties) of the sediment d. the fossil content (if any)

The correct answer is b. all of these things and anything else we can find to establish a sequence of formation

Explanation:

Stratigraphy is responsible for the study of rocks as bodies of three-dimensional extension to determine their extension and sequence; and as a consequence, determine the order and order and temporality of the events of the terrestrial history recorded in the sediments. Sedimentary rocks have accumulated an enormous amount of stratigraphic data that is observable on exposed surfaces (natural outcrops), excavations, quarries, mines, well drilling, etc., and it is up to the stratigraph to organize and integrate this accumulation of information. Stratigraphy records in rocks: forms, lithological compositions, physical and geochemical properties, original successions, age relationships, distribution and content of fossils; All these features serve to recognize and reconstruct geological events sequentially.

81 is a composite number.