单选题 A high life expectancy coupled with a high crude birthrate makes it difficult to increase per capita GNP. Finally, people have different views on what is the proper rate of population. Some feel that the earth is too crowded already and that societies should work for zero population growth—the condition in which the average number of births and deaths balances so that a population stops growing.
A. makes it difficult
B. what
C. that
D. balances