How Did William Smith Produce a Complete Picture of the Layers

How did William Smith produce a complete picture of the layers in a deep section of local rocks when the layer was not actually exposed in any one place?

A) He made inferences based upon his observations and knowledge of a complete section of exposed rock 150 km away.
B) He utilized the chemical compositions of rocks that were brought up by a well-driller's core sampler.
C) He drew correlations between different parts of the deep section that were exposed in nearby locations.
D) He applied his knowledge of the past history of the local geology and matched up the fossils.