Which changes by the largest factor along the main sequence from spectral types o to m—mass or luminosity?

Answer(b) both involve the use of levers;

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.

The  moon has  insufficient gravity to retain an  atmosphere. we conclude that the moon has no atmosphere  because its gravitational field is too weak to  have  retained any  atmosphere  that it had over times comparable to the age of the solar system.