Passage 2: Questions 31-40
To improve the productivity of land, farmers in developed countries use enormous quantities of
artificially produced nitrates as agricultural fertilizers. There is now growing evidence that these
__(31)__ nitrates are overloading the natural system. __(32)__ they can be broken down or
converted into atmospheric nitrogen, they are often leached out of the soil by rain. These dissolved
nitrates are often carried into streams and river systems, and down into ground water. This is the
time when nourishment can mean death. In some places, water for domestic use contains __(33)__
high concentrations of nitrates that it exceeds safety levels for human consumption. Excessive
quantities of nitrates reaching the water system can cause an algal bloom, a sudden and dramatic
__(34)__ in the populations of algae, which use up the oxygen in the water. This change __(35)__
leads to death of much of the life in the lake.
To cope with such danger, people may need to work with nature. In the world’s tropical
regions, __(36)__ the temperature is generally high, the bacteria that cause denitrification can
thrive. They can impoverish the soil and the plant life by __(37)__ the nitrates very quickly. The
paddy field system overcomes this __(38)__ by waterlogging soil and slowing down the action of
the denitrifying bacteria. Flooded fields also support the growth of blue-green algae, which are able
to __(39)__ nitrogen from the atmosphere into nitrogen compounds, making it available to the
growing crop. __(40)__, farmers in the tropics have been able to use land productively, maintaining
the fertility of the soil under the difficult conditions at these latitudes.
【題組】35. (A) in turn (B) on purpose (C) without reason (D) for example (E) no longer
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