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.

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