2022 AMC 10B Problem 15

Let S_n be the sum of the first n terms of an arithmetic sequence that has a common difference of 2. The quotient \frac{S_{3n}}{S_n} does not depend on n. What is S_{20}?

\textbf{(A) } 340 \qquad \textbf{(B) } 360 \qquad \textbf{(C) } 380 \qquad \textbf{(D) } 400 \qquad \textbf{(E) } 420

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