Why are urban populations easy to get diseases from food, according to the last paragraph?

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks. 

     Coincident with concerns about the accelerating loss of species and habitats has been a growing appreciation of the importance of biological diversity, the (18) ______ of species in a particular ecosystem, to the health of the Earth and human well-being. Much has been written about the diversity of terrestrial organisms, particularly the exceptionally rich life associated tropical rainforest habitats. Relatively little has been said, (19) ______, about the diversity of life in the sea even though coral reef systems are comparable to rain forest in terms of richness of life. 

     An alien exploring Earth would probably (20) ______ priority to the planet's dominants – most distinctive feature - the ocean. Humans have a bias toward land (21) ______ sometimes gets in the way of truly examining global issues. Seen from far away, it is easy to realize that landmasses occupy only one-third of the Earth's surface. Given that two thirds of the Earth's surface is water and that marine life lives at all levels of the ocean, the total three-dimensional living space of the ocean is perhaps 100 times (22) ______ than that of land and contains more than 90 percent of all life on Earth even though the ocean has fewer distinct species.

Mark the letter A, B, C, or D on your answer sheer to indicate the corrected answer to each of the following questions 

Mark the letter A, B, C, or D on your answer sheer to indicate the option the best completes the following exchanges