(B) says, "A newspaper works to prevent Party A from winning a majority of seats in the legislature by publishing editorials defending candidates from a rival party against attacks by certain broadcast journalists."
We're looking for the answer that is most analogous to the process described in the last paragraph (stopping the spreading of thistles). In the passage, the study examines spreading beneficial organisms into overproduced areas of land in order to help defend native plants from various disease-causing organisms and to also slow the spreading of problematic weeds, such as thistles. The basic gist of the process is that they're spreading the defending organisms in order to weaken the disease-causing ones. With (B), the newspaper is working against political party A by publishing editorials that defend their rival candidates (this is analogous to spreading the defenders) against attacks from other journalists (this is analogous to weakening the disease-causing organisms).
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