A hominin (human family) skull was found in a shale layer between two fine-grained igneous rock layers. There were no baked zones on the rocks above the igneous layers. The skull had been 100% fossilized and changed to stone; there was no original skull material left. On the basis of this information, decide which of the following statements is true.

a. The skull should be dated by the carbon-14 method, since this is the method used for organic material older than 1 million years.
b. The igneous layers can be dated by radiometric methods; the shale layers cannot be dated radiometrically.
c. The skull itself cannot be dated, because it is no longer organic material and it is not igneous material. The age of the skull is determined by its position between the igneous layers.
d. The igneous layers must be lava flows, because they are fine-grained rock and there are no baked zones on the rock above them.