[单项选择]Increase in the temperature of tropical soil ______.


A. might cause the overflow of flood
B. may lead to the loss of organic nitrogen in it
C. will produce a great deal of organic nitrogen
D. is harmless to the growth of rain forests

Too much metal in the soil is considered pollution. Yet too little metal in a person’s diet can cause health problems. An American scientist says the answer to both problems may be plants that take up large amounts of metal in their tissues.
David Salt is an expert on plants at Purdue University in West Lafayette, Indiana. He is working with plants that store large amounts of metals.
In recent years, scientists have shown interest in using such plants to clean up harmful waste materials, Professor Salt says such plants also could be used to improve people’s diets or even to create foods that fight cancer.
Humans need a number of metals in their diets, including iron and zinc. These metals are needed in very small amounts, however. That is why they are called micronutrients. The lack of micronutrients is blamed for health problems in many developing countries. Children and pregnant women are mostly affected.
Recently, Mr. Salt an
A. copying tissues from wild plants that take up selenium naturally into crop plants
B. copying genes that causes the plants to take up selenium from the soil
C. moving selenium from the tissues of wild plants into crop plants
D. moving genes from wild plants that take up selenium naturally into crop plants