<em>The correct answer is option B i.e. Nitrates.
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<u>Explanation:</u>
1. The golf courses, or cricket and footballgrounds need a thick sheet of grass growing on them for their perfect performance. Farms do need grasses for animals.
2. Acid is not poured on the grasses to grow. Some nitrates poured on grass may convert into nitric acid, but its not significant.
3. Fuel and sewage are also not poured on golf courses or farms to grow grasses.
4. But nitrates are good fertilizers which leads to growth of grasses and plants, needed for farms and golf courses.
By cooling the heart, its need for oxygen is reduced. This allows surgeons to work on the heart for two to four hours without damaging the heart tissue.