2011 AMC 10A Problem 6

Set A has 20 elements, and set B has 15 elements. What is the smallest possible number of elements in A \cup B, the union of A and B?

\textbf{(A)}\ 5 \qquad\textbf{(B)}\ 15 \qquad\textbf{(C)}\ 20\qquad\textbf{(D)}\ 35\qquad\textbf{(E)}\ 300

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.