I'm really confused after facing with the answer explanation...
can someone explain me with answer C is not correct when in the question is clearly stated that she swim the same distance at the same speed?
I also picked C, because I was thinking that the water flow would cancel out between helping the swimmer one way and then slowing the swimmer down the other. but this thinking is WRONG. I think it's most similar to thinking that a 50% gain from 100 will be the same as a 50% loss from 150. Once you understand that you need to take the speed of the current into account, the rest of the solution falls into place.
As the solutions says, the best way to answer this question is by plugging in numbers,
Current - 2km/h
Swimmer speed - 4km/h
River length - 12km
during the rapid water, going downstream (ie with the current), we add the speed of the swimmer and the current to get 6km/h
going upstream, we subtract the current's speed from the swimmers speed which gives us 2km/h
R x T = D
downstream - 12km/6km/h=2 hours
upstream - 12km/2km/h = 6 hours
so with the current it's a total of 8 hours
Now, without the current it's much easier. we just assume a total of 24km at a speed of 4km/h
24km/4km/h = 6 hours
X > Y, hence A
hope that helps.