Nuclear radius:
\[
R\propto A^{1/3}
\]
Given:
\[
R_1:R_2=2:1
\]
So mass numbers:
\[
A_1:A_2=2^3:1^3=8:1
\]
Since the nucleus was initially at rest, momentum is conserved:
\[
m_1v_1=m_2v_2
\]
Thus,
\[
v_1:v_2=m_2:m_1=1:8
\]
So the direct calculation gives
\[
\boxed{1:8}
\]
which corresponds to option (C), not (E).