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 conventional spacecraft equipped with the device would not be appreciably more expensive to construct than current spacecraft typically are.', "The device for manufacturing fuel from the Martian atmosphere would not take up any of the spaceship crew's living space.", '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.', 'The fuel manufactured from the Martian atmosphere would not differ in composition from the fuel used to travel to Mars.']