The answer is, "B", "Ammonia".
The average speed of the bus from Lansing to Detroit is

while the average speed of the bus from Detroit to Lansing is

The distance covered by the bus in the two trips is the same (the distance between the two cities), therefore, the average speed of the round trip can be calculated as the mean of the two speeds:
Solar system is the gravitationally bound system that consists of the sun and the objects that orbit around it directly or indirectly. These objects includes the planets which orbit the sun directly an other small objects such as meteoroids, asteroids, satellites of the planets and numerous comets. The sun makes up most of the solar system' mass.