Determine whether or not price discrimination occurs in each of the following situations; if it does, indicate the reasons why discrimination is advantageous and how customers are segregated:
a. family fares on airlines and railroads;
b. lower theater admissions for children or senior citizens than adults;
c. lower fares for regular commuters on railroads and bus lines;
d. higher prices for American subscribers of foreign periodicals than those charged residents of the countries (the differences exceeding differences in postal charges).