12. 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. Therefore, 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) The amount of fuel needed for a spacecraft to return from Mars is the same as the amount of fuel needed to travel from Earth to Mars.

(B) The fuel manufactured from the Martian atmosphere would not differ in composition from the fuel used to travel to Mars.

(C) The device for manufacturing fuel from the Martian atmosphere would not take up any of the spaceship crew’s living space.

(D) A conventional spacecraft equipped with the device would not be appreciably more expensive to construct than current spacecraft typically are.

(E) 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.

答案是E。

做题的时候觉得五个好象都有问题，最后蒙了个A，但知道same as有缺陷。但是E的推断实在是搞不清，还清NN指点

我第一次读题时，想到这个device是个空中加油设备，呵呵，想多了。

还是有些怪，我理一下思路

假设从地球到火星需要燃料A，从火星到地球需要燃料B，如果不带Device，则理论上不可

行。

因为　A＋B　can not meet all journeys and A+B is too heavy,it must be A+B+a.

But so much fuel can not be reachable.

so we need this device.

the required fuel weight is A+b. and surely b<B is supposed to be met.

if we want to conclude that device weight < B weight, we need to depend on

this assumption that addition any equipment weight equals weight of the fuel

that the equipment requires.

and we inspect this sectence: 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.  It seems that weight of required fuel > (weight of craft

+ weight of required fuel) need to come into existence, but of course that

condition is impossible. But how I cant get E conclusion.

I still can not get this conclusion.

I'm confused.

这个题还是比较清楚的。

假如地球去火星所需燃料为A,而回程所需燃料为B,那么A+B的重量是飞行器承受不了的。

假如要用Device（简称D)，那么A+D的重量必须小于A+B的重量，飞行器才承受得了。也就

是D<B.

哦，明白了， be launched from earth

钻牛角尖了