A．about
B．would
C．round
D．outside

A．Recently, two American astronauts have landed on Mars.
B．The surface of Mars is very hot.
C．The journey to the moon started the future exploration in space.
D．There is life in Venus.

A．carrying passengers to planets
B．acting as stop- over points
C．refueling spacecraft
D．providing supply of air, food and water