The most frequently used portal of entry for pathogens is the

Mucosal surfaces of respiratory and gastrointestinal tract

Explanation:

A portal of entry is an avenue through which pathogenic microbes can have access to its host tissue. Since the pathogens are from the external environment, the portal of entry must be a site connected to the external.

Surfaces of the mucous membranes like respiratory tract, gastrointestinal tract etc. links the external and internal environment, hence, they are the most employed portals of entry by pathogens.

Also, the respiratory and gastrointestinal tract are vulnerable portals of entry because particles comprising of microbes are constantly taken in via the nose and mouth respectively.

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.

links? do you have a link? i don't see anything?