What does it mean when a compound’s chemical formu does not include any numbers

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

for a different answer set the answer would be because the oxygen in the water had depleted (apex)