PrepTest 104, Section 4, Question 9
The only physical factor preventing a human journey(80 more words not shown)
Which one of the following(8 more words not shown)
- AThe amount of(24 more words not shown)
- BThe fuel manufactured(17 more words not shown)
- CThe device for(17 more words not shown)
- DA conventional spacecraft(17 more words not shown)
- EThe device for(29 more words not shown)
Reading the argument
If carrying all the fuel makes the spacecraft too heavy to launch, could we send it up in pieces and assemble it in space? That is one possible response to the problem. But the author proposes a different solution: carry a device that makes the return fuel from Mars's atmosphere, instead of carrying that fuel from Earth.
How much does the device weigh?
The entire obstacle is weight, so replacing fuel with a machine only helps if the machine reduces what we have to launch. If the machine weighs as much as the return fuel, we've traded one heavy load for another. If it weighs more, we've made the problem worse.
We could also ask whether the machine produces fuel fast enough for the planned trip. A machine that eventually works might still be impractical if it takes hundreds of years. But the weight comparison is the most direct concern raised by this proposed solution.
What we’re looking for
For the assumption question, look for something the author needs in order to say this device makes the trip possible. In particular, it must save weight compared with carrying the return fuel. Accept the passage's claim that weight is the physical obstacle; we don't need to import other real-world problems with space travel.
Working the answers
- A
The two legs of the trip don't have to require exactly the same amount of fuel. The return could require somewhat more or somewhat less, provided the device can supply the necessary return fuel and the craft can be launched. Equality isn't what makes the proposed solution work.
- B
The fuel needs to work for the return trip; it need not have exactly the same composition as the outbound fuel. A usable fuel could differ in composition while still serving the purpose described. The argument depends on getting the needed fuel on Mars, not on chemical identity between the two supplies.
- C
Taking up some living space doesn't automatically make the trip impossible. Perhaps the crew can manage with a smaller exercise area or less room to spread out. We aren't told the device would leave too little space for survival. The absolute claim that it takes up none of the crew's living space is stronger than the argument needs.
- D
The proposed solution concerns a physical obstacle, not whether the spacecraft can be built for roughly the current price. The device might make the craft much more expensive and still solve the weight problem. We don't have a budget limit that makes unchanged construction costs a necessary assumption.
- ECorrect
The device has to weigh less than the return-fuel load it replaces. Otherwise, leaving that fuel behind and adding the device wouldn't reduce the launch weight, and the stated obstacle would remain.
This is necessary, but it need not be enough by itself. A device only slightly lighter than the fuel might still leave the craft too heavy. The point is that the plan cannot solve this weight problem unless the substitution saves some weight. E states that required first step without claiming that every lighter device guarantees a successful round trip.
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