The only physical factor preventing a human journey to Mars has been weight. Carrying enough fuel to propel a conventional spacecraft to Mars and back would make even the lightest craft too heavy to be launched from Earth. A device has recently been invented, however, that allows an otherwise conventional spacecraft to refill the craft' s fuel tanks with fuel manufactured from the Martian atmosphere for the return trip. Ttherefore, it is possible for people to go to Mars in a spacecraft that carries this device and then return. Which one of the following is an assumption on which the argument depends?
A: A conventional spacecraft equipped with the device would not be appreciably more expensive to construct than current spacecraft typically are.
B: The device for manufacturing fuel from the Martian atmosphere would not take up any of the spaceship crew's living space.
C: The device for manufacturing fuel for the return to Earth weighs less than the tanks of fuel that a conventional spacecraft would otherwise need to carry from Earth for the return trip.
D: The fuel manufactured from the Martian atmosphere would not differ in composition from the fuel used to travel to Mars.
Choose A, B, C or D as your solution.
C