According to the lecture, international politics caused many Puerto Ricans and Cubans to move to the U.S. in the 1950s and 1960s. The correct option among all the options given in the question is option "c. At first, just a few Puerto Ricans migrated to the United States of America. Later on the flow of the immigrants increased.
Part of it was because of the Industrial Revolution and the invention of the cotton gin by Eli Whitney.
<span>Slate was more common in wet microclimates.</span>
