AMC 10B 2022 (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

\dfrac{S_{3n}}{S_n}

does not depend on

n

. What is

S_{20}

\text{(A)}\;340

\text{(B)}\;360

\text{(C)}\;380

\text{(D)}\;400

\text{(E)}\;420