A．it is nature itself that should take the blame for the decline of North Africa
B．climatic changes coupled with forest abuse result in the decrease of forests in North Africa
C．without sufficient forests, the rains will take away all the organic nitrogen
D．since Roman times the rain forests in North Africa has been decreasing gradually

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