Two atoms have two different proton counts. This means that the atoms a. belong to two entirely different elements.

Selected:b. represent different isotopes of one element. This answer is incorrect.

c. are likely to react violently with each other.

d. might be different ions of the same elements.

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).

the correct answer is option d.

an appositive is a noun, noun phrase or noun clause which follows another noun or noun phrase in order to describe it or name it another way. they are usually used between commas, brackets or dashes.

the example sentence with the correct use of commas is the following:

her uncle, the man over there in the red shirt, owns the hardware store in town.