Which of the following, if true, best completes the argument below
Comparisons of the average standards of living of the citizens of two countries should reflect the citizens’ comparative access to goods and services. Reliable figures in a country’s own currency for the average income of its citizens are easily obtained. But it is difficult to get an accurate comparison of average standards of living from these figures, because （）.
A．there are usually no figures comparing how much of two different currencies must be spent in order to purchase a given quantity of goods and services
B．wage levels for the same job vary greatly from country to country, depending on cultural as well as on purely economic factors
C．these figures must be calculated by dividing the gross national product of a country by the size of its population
D．comparative access to goods and services is only one of several factors relevant in determining quality of life
E．the wealth, and hence the standard of living, of a country’s citizens is very closely related to their income