Which three of these classes of animals practice internal fertilization?

1) mammals
2) birds
3) amphibians
4) reptiles
5) arthropods

The mammals, birds and reptiles are the three classes in the animal kingdom that practice the internal fertilization. These three classes, that is the birds, mammals, and the amphibians, always use fertilizes the egg internally. The amphibians on the other hand fertilizes externally. The arthropods, on the other hand, reproduce both by external fertilization and internal fertilization.

Hence, the answer is 'mammals, birds, and reptiles'.

it has one origin of replication, but it proceeds in two directions - d)

water is a polar molecule. a chemical bond is a force of attraction between atoms or ions. bonds form when atoms share or transfer valence electrons. atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons. so, false

explanation:

in h2o molecule, two water molecules are bonded by a hydrogen bond but the bond between two h - o bonds within a water molecule are covalent. the greater the electronegativity difference, the more ionic the bond is. bonds that are partly ionic are called polar covalent bonds. nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.