When does a fetus get its heart beat?

How quickly a drug enters your bloodstream also depends on

Group of genes that encode molecules found in all body cells, creating a biochemical “fingerprint” that serves as the “id” for cells so that they are marked as “self” a) deoxyribonucleic acid (dna) b) entry inhibitors c) major histocompatibility complex (mhc) d) non-nucleoside reverse transcription inhibitors (nnrtis)

How could natural selection best be used to explain the evolution of the white fur of arctic fox