Studies show farmland in Africa is often lacking in important nutrients. But researchers say combining different farming methods may help. Since the world food crisis several years ago, researchers have directed more of their attention to small farms. Most farms in areas south of the Sahara Desert are only about one or two hectares. One of the goals is to increase production without necessarily clearing more land to grow additional crops. American researchers say that can happen with greater use of an agricultural system called perennation. It mixes food crops with trees and perennial plants - those that return year after year. Mr. Reganold, who is with Washington State University, says poor soil may have resulted from years of weathering that washed away many nutrients. He says some farmers may have done more harm than good. He estimates that up to two billion dollars worth of nitrogen, phosphorous and potassium is lost from African soil each year. The scientist says the word perennation defines three systems that are already used in Africa. The oldest of the three is called evergreen agriculture. This is where farmers plant trees with their crops. John Reganold says farmers in Africa have been doing this for sixty years, but it seems to be growing in popularity. The method is gaining widespread use in countries such as Niger, Burkina Faso, Malawi and Zambia. The trees are planted among maize, millet or sorghum crops. They not only add nitrogen to the soil through their roots, but also through their leaves when they fall off and break down. At other times of the year, the trees can protect plants from strong sunlight. Mr. Reganold was one of three researchers who wrote a report about perennation. It was published in the journal Nature. Researchers focused their attention on small farmlands for the reason that    _   .
Africa was going through a food crisis most farms in areas south of the Sahara Desert are small there are not enough farmlands farmers may grow additional crops on farmland
Question: Studies show farmland in Africa is often lacking in important nutrients. But researchers say combining different farming methods may help. Since the world food crisis several years ago, researchers have directed more of their attention to small farms. Most farms in areas south of the Sahara Desert are only about one or two hectares. One of the goals is to increase production without necessarily clearing more land to grow additional crops. American researchers say that can happen with greater use of an agricultural system called perennation. It mixes food crops with trees and perennial plants - those that return year after year. Mr. Reganold, who is with Washington State University, says poor soil may have resulted from years of weathering that washed away many nutrients. He says some farmers may have done more harm than good. He estimates that up to two billion dollars worth of nitrogen, phosphorous and potassium is lost from African soil each year. The scientist says the word perennation defines three systems that are already used in Africa. The oldest of the three is called evergreen agriculture. This is where farmers plant trees with their crops. John Reganold says farmers in Africa have been doing this for sixty years, but it seems to be growing in popularity. The method is gaining widespread use in countries such as Niger, Burkina Faso, Malawi and Zambia. The trees are planted among maize, millet or sorghum crops. They not only add nitrogen to the soil through their roots, but also through their leaves when they fall off and break down. At other times of the year, the trees can protect plants from strong sunlight. Mr. Reganold was one of three researchers who wrote a report about perennation. It was published in the journal Nature. Researchers focused their attention on small farmlands for the reason that    _   .
A. Africa was going through a food crisis
B. most farms in areas south of the Sahara Desert are small
C. there are not enough farmlands
D. farmers may grow additional crops on farmland
Answer: