Read the passage carefully, then choose the best answers:

The dramatic growth of the world's population in the twentieth century was on a scale without parallel in human history. Most of this growth had occurred since 1950 and was known as the population 'explosion'. Between 1950 and 1980 the world population increased from 2.5 to over 4 billion, and by the end of the twenty century the figure had risen to about 6.6 billion. Growth of this size cannot continue indefinitely. Recent forecasts suggest that the total population will level out at between 10 and 15 billion in the mid twenty first century. Already there are encouraging signs that the rate of increase in many less developed countries is beginning to slow down.

According to the passage, at no period in human history has there been ______.

A. a population explosion of the magnitude as the one in the twenty century.

B. a universal fear about the future of man.

C. as comprehensive a study of population problems as the one envisaged now.

D. so much consensus among nations concerning the population of the world.