Reading Comprehension

The tsetse fly, belonging to any of approximately twenty species
composing the genus Glossina, is indigenous to Africa and is found
primarily in forests and savannahs south of the Tropic of Cancer.
Dependent on vertebrate blood for nourishment, the tsetse fly is
equipped with a long proboscis which is sharp enough to penetrate most
animal skins and powerful enough to enable the tsetse to drink quantities
of blood up to three times its own body weight.

At the same time that the tsetse drains blood, it can also transmit a
variety of dangerous diseases. A bite from a tsetse fly can induce African
sleeping sickness in human beings and nagana, a similar ailment, in
domestic livestock. The agent of these diseases is the trypanosome, a
unicellular, flagellated parasite which feeds primarily on the blood of
vertebrates and is generally transmitted by an intermediary leech or
insect host, such as the tsetse fly. In humans the trypanosome causes
damage to the brain and spinal cord, leading to extreme lethargy and,
ultimately, death; in livestock, trypanosomes destroy red blood cells,
causing fatal anaemia.

The immune system is ill-equipped to counter trypanosomes. As the
immune system attempts to counter disease, antibodies are produced to
attack microbes whose antigens, surface proteins, are foreign to the
body. However, the trypanosome is capable of disguising itself by altering
its genetic code, thereby changing its antigen coating in resistance to
each new antibody that evolves. This "•quick change"– has confounded
pathologists and made the development of effective vaccines elusive.

A controversy has been sparked between proponents of the
elimination of the tsetse fly and African environmentalists. Those in
favour of eradication feel that in addition to reducing disease, the
removal of the tsetse fly will open immense tracts of land to cattle
breeding. This, however, is precisely what the opposition fears.
Environmentalists and conservationists dread the day when cattle and
livestock, permitted to roam and graze freely, will uncontrollably devour
plush African grasslands, converting them into barren desert. They argue
that the tsetse fly must remain for the sake of the land.

With efforts to eradicate the tsetse fly largely unsuccessful, control
may offer the only available option for the interests of both health and
environment. Since the protozoan cannot be conquered through
antibodies or vaccines, scientists have begun efforts to prevent the
transmission of the trypanosome parasite by eliminating the tsetse.
Attempts to eradicate the tsetse fly, however, have met with little
success. Rhodesia used to combat tsetse by extensive brush cleaning,
game shooting, and chemical attack, yet the fly persisted. Aerial pesticide
treatments have produced inconclusive results.

The reproductive cycle of the tsetse fly is such that a larva pupates
underground for several weeks before it emerges as an adult fly. This
makes repetitive chemical sweeping at intermittent periods an
inconvenient necessity. All of these methods, however, share the
weakness of dependence on harmful chemicals, such as DDT, which
threaten both the health of the humans who handle them and the
environment in which their toxic residues amass.

What is the primary purpose of the fourth paragraph in the passage
A. to decsribe the harmful effects of the tse tse fly
B. to argue that the proliferation of tse tse flies can lead to large scale
deforestation of African grasslands
C. to discuss a beneficial impact of tse tse flies
D. to state that efforts to eradicate the tse tse flies have generally proved
to be ineffective
E. to discuss the reproductive cycle of a tse tse fly