Critical Reasoning

The use of radar detectors in commercial vehicles was banned because commercial truck and bus drivers were using these devices to drive faster than the posted speed limit without fear of arrest. Since drivers of noncommercial vehicles also use radar detectors and since speeding decreases safety for any vehicle, use of radar detectors should also be banned in noncommercial vehicles to increase safety.
Source: Gprep:

Which of the following, if true, most strongly supports the argument above?
A. The average noncommercial-vehicle driver is involved in less long-distance driving than is the average commercial-vehicle driver.
B. In many places airplanes or helicopters are used rather than radar to locate vehicles traveling faster than the posted speed limit.
C. The ban on radar detectors in commercial vehicles has been effective in deterring them from speeding.
D. Traffic accidents involving a truck or bus generally pose a greater threat of injury or death than do other accidents.
E. Radar detectors are sometimes set off by devices other than the law enforcement devices that the radar detectors were built to detect.

OA [spoiler]C[/spoiler]

i think that c is assumption of the argument;
question: does an assumption always strengthen the argument?

hoji wrote: question: does an assumption always strengthen the argument?

To be credible, the argument should always replace all assumptions with facts. That is the difference between critical thinking and propaganda. Propaganda feeds off of personal biases. Most (even critical thinkers) will often use their own biases as if fact.

Assumption C discusses two 'nouns' found in the argument. "Ban on radar detectors in commercial vehicles" and "deterring them from speeding". (Other assumptions introduce 'nouns' not found in the argument.)

The argument defines no relationship between both nouns. The 'verb' "has been effective" is in the assumption. That assumption necessary to complete the argument.

The argument only implies a relationship (maybe or maybe not) between speeding and use of radar detectors. Radar detectors might cause speeding (A causes B). Or radar detector and speeding is coincidental (A and B exist independently). Or speeding causes use of radar detectors (B causes A).

To make the original argument valid, a reader must add an assumption. That radar detectors cause, encourage, or make possible speeding (A causes or results in B). None of those 'verbs' are defined or even exist.

A reader will often make that assumption using personal biases. Biases can come from anywhere including hearsay in a newspaper article or from racism. Did racism cause one to create that assumption. Probably not. But probably (maybe or maybe not) is not 'no'.

In this case, the argument is missing a necessary and hard fact. That radar detectors cause or result in speeding. Without assumption C, then the argument loses credibility.

hoji wrote: i think that c is assumption of the argument;
question: does an assumption always strengthen the argument?

Please note there is a difference between strengthening the argument and supporting the argument.

We generally categorize assumption in two ways :
- assumption in a supporter role.
- assumption in a defender role.

From first type, we can consider that the assumption do support the argument in some way or the other. But assumption do not work in the same way as strengthener works.

An assumption : must be true for the argument to be true
an strengthener : add something to the argument, but the argument can stand without this.

So in CR, Strengthening is not same as support or defend.

Few days back, I was quiet confused on this. Posted something on this concept. Please take a look at the experts explanation on it.

https://www.beatthegmat.com/assumption-i ... 00004.html


Hope this helps !!

(C) is the only option supporting the conclusion.

An assumption can be a strengthener , however the reverse isn't true. A strengthener is something that just makes the conclusion more likely to be true. However an assumption is something that if isn't true, invalidates the argument.
Here the conclusion says that the ban on usage of radar in non commercial vehicles will also increase the safety.
So the most basic assumption here springs from the word "ALSO". The argument assumes that the ban was effective in deterring the commercial vehicles from using radar.

(E) is not a strengthener here. "Sometimes" doesn't reveal the frequency of the error performed by radar equipped Non commercial vehicles. It can be 1 in 100 or 51 in 100. So we don't know.

Thanks
Prashant