After the bite of the tsetse fly, parasites multiply in the blood of animals and
a. human’s blood
b. of humans

option B, of humans

Explanation:

Of human is correct because in the structure of sentence blood word is used preceding to the organism to which it belongs.  Thus , a connecting word is required to establish a relationship between blood and human. Therefore "of " is used before blood to connect it with the organism to which it belongs.

If blood has not been used earlier in the sentence , option A would have been correct.  

considering all explanations above option B is the right choice of answer.

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.