Which of the following factors is a significant reason U.S. business professionals often need to understand the cultures of other countries?

a. Recent changes to government regulations require cultural education before companies are granted export licenses.

b. The U.S. economy has been shrinking for the past 20 years, forcing companies to look overseas.

c. Many countries require business executives to be fluent in at least two languages.

d. Thousands of U.S. companies, including many of the largest corporations in the country, rely on markets in other countries for a significant portion of their sales.