For each of the following pairs of variables, give your best guess (and an explanation) of whether the variables have a positive, negative, or roughly zero correlation in a cross-section of countries:

a. GDP per capita and the number of books printed per capita

b. GDP per capita and the fraction of the population suffering from malnutrition

c. Fraction of the population that wears eyeglasses and life expectancy

d. Number of automobiles per capita and number of letters in the country’s name