Two atoms appear to have the same mass number. these atoms
a) must have the same atomic number.
b) must have the same number of electrons.
c) must have the same chemical properties.
d) must have the same number of protons + neutrons.
e) must have the same atomic number, the same number of protons + neutrons, the same number of electrons, and the same chemical properties.

one characteristic that is shared by all is a soft body. many

of these invertebrate animals also have an outer shell to protect their

body. oysters, clams, snails, and mussels are all mollusks. so are octopuses, squids, and slugs. mollusks live on land and in freshwater.

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).