In Recent Years the Annual Productivity of Slash-And-Burn Systems Has

In recent years the annual productivity of slash-and-burn systems has tended to decline over that achieved by the traditional shifting cultivation systems of the past.To what is this decline most likely due?

A) decrease in the length of the fallow period
B) increase in the period of food production in a given cycle
C) failure to grow legumes in the cropping system
D) reduction in the C/N ratio of the soil organic matter
E) reduction in the use of fertilizers