Farmers can produce more by planting several different crops in the same field.But their success depends on which crops they plant together and how they mix   them in the filed.In the past, the only way the farmers could learn which crops grow best together is to plant them and wait for the results.Now, however, a computer can give them answers.A new computer program written by Dr John Vanderwell of Michigan University can tell a farmer which plants grow well together and which do not.It can tell him how to plant the different crops; if he should plant each in a separate row, or mix the crops in the same row.It can tell the farmer how changes in planting each of the crops will affect the production of all of them.And it can tell him which plants can help reduce losses   from diseases. According to the passage, to get the best result, one needs to choose carefully   _  .
the kinds of crops to be planted together the way for different crops to be mixed both A and B either A or B
Question: Farmers can produce more by planting several different crops in the same field.But their success depends on which crops they plant together and how they mix   them in the filed.In the past, the only way the farmers could learn which crops grow best together is to plant them and wait for the results.Now, however, a computer can give them answers.A new computer program written by Dr John Vanderwell of Michigan University can tell a farmer which plants grow well together and which do not.It can tell him how to plant the different crops; if he should plant each in a separate row, or mix the crops in the same row.It can tell the farmer how changes in planting each of the crops will affect the production of all of them.And it can tell him which plants can help reduce losses   from diseases. According to the passage, to get the best result, one needs to choose carefully   _  .
A. the kinds of crops to be planted together
B. the way for different crops to be mixed
C. both A and B
D. either A or B
Answer: