Why could you use radiometric dating to date the igneous rock layers, but not the sedimentary rock layers?

"Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years. So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil."

Explanation: (i found this but i do not know if it helps)

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.