= Solution
An <A-stable> method must satisfy $|P_s(z)|\le1$ for every $z$ with nonpositive real part. A consistent explicit <Runge-Kutta method> has $P_s'(0)=1$, so its stability <polynomial> is nonconstant. Along the negative real axis the leading term of any nonconstant <polynomial> has magnitude tending to infinity. Hence
$$
\boxed{\text{No consistent explicit Runge-Kutta method is A-stable}.}
$$
The positive order assumption in the question supplies the required consistency.
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