Question

how is radioactive decay used to date sedimentary rocks?
the amounts of potassium and argon in sedimentary rocks are measured.
the amounts of carbon - 14 and nitrogen in sedimentary rocks are measured.
the amounts of unstable elements in each layer of the sedimentary rocks are compared.
the amounts of unstable elements in the volcanic layers above and below the sedimentary layers are measured.

Explanation:

Sedimentary rocks are made up of sediments from other rocks. Radioactive dating is more reliable for igneous rocks (like volcanic layers). To date sedimentary rocks, we measure the unstable elements in volcanic layers (igneous) above and below the sedimentary layers. Carbon - 14 is used for recent organic materials (not old sedimentary rocks). Potassium - argon dating is for igneous rocks too, but the key is that sedimentary rocks themselves are not directly dated by measuring their own unstable elements (as they are composed of sediments).

Answer:

The amounts of unstable elements in the volcanic layers above and below the sedimentary layers are measured.