2. A river has a constant flow rate of 3 km per hour. A boat travels 4 km downstream and then returns to the starting point at a constant speed, taking a total of 1 hour (ignoring the time to turn the boat around). What is the ratio of the boat's downstream speed to its upstream speed?
(A) $5: 2$
(B) 2:1
(C) $5: 3$
(D) 3,2
2. Let the speed be $x$, then
$$
\frac{4}{x+3}+\frac{4}{x-3}=1 \text{.}
$$

Solving it, we get $x_{1}=9, x_{2}=-1$ (not suitable, discard) At this point $\frac{x+3}{x-3}=\frac{12}{6}=2$, that is, the ratio of the boat's speed downstream to upstream is 2:1.
Choose $\mathrm{B}$.