A3. For which $n$ does the equality $3^{2002}-3^{2001}+3^{2000}-3^{1999}=n\left(3^{1999}\right)$ hold?
(A) 18
(B) 19
(C) 20
(D) 21
(E) no such $n$ exists
A3. If we factor out $3^{1999}$ on the left side of the equation, we get $3^{1999} \cdot\left(3^{3}-3^{2}+3-1\right)=20 \cdot 3^{1999}$.