Most traditional sources of energy come from

a.) biosphere
b.) hydrosphere
c.) lithosphere
d.) atmosphere

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.

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.