Which have scientists learned from studying seismic waves that travel through earth’s interior?

a. the outer core of earth is solid.

b. they have learned the precise depths where the boundaries between the earth's layers are located.

c. the inner core of earth is liquid.

d. earth's mantle is liquid.

"They have learned the precise depths where the boundaries between the earth's layers are located" is the one among the following choices given in the question that scientists have learned from studying seismic waves that travel through earth’s interior. The correct option among all the options given is option "B". 

Distance covered = speed x timethe distance traveled going against the stream = distance traveled with the stream.therefore it is the speed and the time that are the variables.as he travels the same distance both ways, speed x time is the same for each way.going upstream his speed is 4 kph; going downstream his speed is 6 kph.therefore we have two simultaneous equations: time upstream x 4 = time downstream x 6time upstream + time downstream = 60 minutestherefore we have the equation: time upstream = 6/10 hour, time downstream therefore = 4/10 hour.distance traveled at 4km/hr in 6/10 hour = 24/10 = 2.4km.check: distance traveled at 6km/hr in 4/10 hour = 24/10 = 2.4km.