Enzymes present in saliva converts (a) starch into simple sugars

(b) proteins into amino acids
(c) complex sugars into simple sugars
(d) fats into fatty acids and glycerol

b. the carbon cycle ensures living things have a source of hydrogen

i think it is dna, i'm not totally sure but i think it might be.

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

a covalent bond, specfically triple bond