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'.

c. earths continental crust is thicker than its lithosphere.

because, the lithosphere is the outermost layer of the earth/ 100 km thick and is defined by its machanical properties.the lithosphere contains oceanic and continental crust that varies in age and thickness across locations and geologic time. the lithosphere is the coolest layer of the earth in terms of temperature. the term lithosphere should not be confused with the use of "geosphere" which is used to indicate all of earth systems, including the atmosphere, hydrosphere   and biosphere. the surface of the earth can be divided into the continental crust and the oceanic crust.   the continental crust is made mostly of granite while the oceanic crust is made of basalt. the average thickness of the continental crust is 25 miles and the average thickness of the oceanic crust is 5 miles.   three types of rock, igneous, sedimentary, and metamorphic, make up the surface of the earth. that's why 3 option is preferable.