A．Because mixed forests and orchards are the recognized procedures these days.
B．Because trees of same species may cause the spread of disease.
C．Because modern orchards and forests are more specialized.
D．Because trees planted crowdedly do not have enough room to develop their roots.

A．Advances in medical science made disease unable to regulate human population growth any longer in many parts of the world.
B．Medical cures have prolonged life but upset the reproductive age group by increasing many populations.
C．In Australia, the use of the myxomatosis virus hardly controlled the rabbit population with increasing resistance to the disease.
D．It is discovered all strains of virus can parasitize and kill the bacteria without harming the organism that the bacteria has infecte