Recent research has claimed that an excess of positive ions in the air can have an ill effect on people's physical or psychological health. What are positive ions? Well, the air is full of ions, electrically charged particles, and generally there is a rough balance between the positive and the negative charged. But sometimes this balance becomes disturbed and a larger proportion of positive ions are found. This happens naturally before thunderstorm, earthquakes when winds are blowing in certain countries. Or it can be caused by a build-up of static electricity indoors from carpets or clothing, or from TV sets, duplicators or computer display screens. When a large number of positive ions are present in the air many people experience unpleasant effects such as headaches, fatigue  , excessive anger, and some particularly sensitive people feel sick or even suffer mental disturbance. Animals are also affected, particularly before earthquakes, snakes have been observed to come out, rats to flee from their burrows, dogs howl and cats jump about unaccountably. This has led the US Geographical Survey to fund a network of volunteers to watch animals in an effort to foresee such disasters before they hit vulnerable areas such as California. Conversely, when large numbers of negative ions are present, then people have a feeling of well-being. Natural conditions that produce these large amounts are near the sea, close to waterfalls or fountains, or in any place where water is sprayed, or forms a spray. This probably accounts for the beneficial effect of a holiday by the sea, or in the mountains with streams or waterfalls. To increase the supply of negative ions indoors, some scientists recommend the use of ionisers. They claim that ionisers not only clean and refresh the air but also improve the health of people sensitive to excess positive ions. Of course, there are the detractors, other scientists, who dismiss such claims and are skeptical about negative/positive ion research. Therefore people can only make up their own minds by observing the effects on themselves, or on others, of a negative rich or poor environment. After all it is debatable whether depending on seismic readings to anticipate earthquakes is more effective than watching the cat. A high negative ion count is likely to be found  _  .
in a mountainous area. by a water spray in a garden. close to a slow-flowing river. near a pound with a water pump.
Question: Recent research has claimed that an excess of positive ions in the air can have an ill effect on people's physical or psychological health. What are positive ions? Well, the air is full of ions, electrically charged particles, and generally there is a rough balance between the positive and the negative charged. But sometimes this balance becomes disturbed and a larger proportion of positive ions are found. This happens naturally before thunderstorm, earthquakes when winds are blowing in certain countries. Or it can be caused by a build-up of static electricity indoors from carpets or clothing, or from TV sets, duplicators or computer display screens. When a large number of positive ions are present in the air many people experience unpleasant effects such as headaches, fatigue  , excessive anger, and some particularly sensitive people feel sick or even suffer mental disturbance. Animals are also affected, particularly before earthquakes, snakes have been observed to come out, rats to flee from their burrows, dogs howl and cats jump about unaccountably. This has led the US Geographical Survey to fund a network of volunteers to watch animals in an effort to foresee such disasters before they hit vulnerable areas such as California. Conversely, when large numbers of negative ions are present, then people have a feeling of well-being. Natural conditions that produce these large amounts are near the sea, close to waterfalls or fountains, or in any place where water is sprayed, or forms a spray. This probably accounts for the beneficial effect of a holiday by the sea, or in the mountains with streams or waterfalls. To increase the supply of negative ions indoors, some scientists recommend the use of ionisers. They claim that ionisers not only clean and refresh the air but also improve the health of people sensitive to excess positive ions. Of course, there are the detractors, other scientists, who dismiss such claims and are skeptical about negative/positive ion research. Therefore people can only make up their own minds by observing the effects on themselves, or on others, of a negative rich or poor environment. After all it is debatable whether depending on seismic readings to anticipate earthquakes is more effective than watching the cat. A high negative ion count is likely to be found  _  .
A. in a mountainous area.
B. by a water spray in a garden.
C. close to a slow-flowing river.
D. near a pound with a water pump.
Answer: