Although the geological record contains some hints of major meteor impacts preceding mass extinctions, there were man...

Deniann on November 2, 2013

Question 21

What is the difference between answer choice A and B?

3 Replies

Melody on November 6, 2013

The conclusion of the following argument is: "Thus, the geological record suggests that there is no consistent causal link between major meteor impacts and mass extinctions."

Why? Because while there are some hints of major meteor impact preceding mass extinctions, there are many major meteor impacts that were not followed by mass extinctions and many mass extinctions that were not preceded by major meteor impacts.

This is a Strengthen with Sufficient Premise question so we are looking for an answer choice that 100% guarantees the conclusion of the argument.

(A) is CORRECT because it provides us with our missing link. If it is true that a consistent causal link between meteor impacts and mass extinctions requires that all major meteor impacts be followed by mass extinctions, then by invoking the contrapositive we know that there is no consistent causal link. Let's diagram the answer choice:

CC ==> All MMIFME
not All MMIFME ==> not CC

The argument tells us that many major meteor impacts are not followed by mass extinctions (i.e. not All MMIFME), which allows us to conclude with 100% certainty that we do not have a consistent causal link (i.e. not CC).

(B) is incorrect because it does not 100% guarantee the conclusion. Let's diagram to see why:

CC ==> many MMIFME

Now, remember, by definition "many" is "some." Some is at least one, possibly all. Though "many" could be "most," it is not necessarily "most." Therefore, since "many" always encompasses the definition of "some," we regard it as the same as "some." So we can re-write this to be:

CC ==> some MMIFME
none MMIFME ==> not CC

The contrapositive states: if no major meteor impacts are followed by mass extinctions, then there is no consistent causal link.

Remember that the negation of "some" is "none." We know from the stimulus that we do have some major meteor impacts that were followed by mass extinctions, but this is our necessary condition, which tells us nothing else, so answer choice (B) does not guarantee the conclusion.

Hope that was helpful! Let me know if you have any more questions.

Amanda on November 10, 2017

Why is D wrong?

Mehran on November 13, 2017

Hi @sharpen7, thanks for your post.

Answer choice (A) can be diagrammed as:
CC ==> ALL MMIFME
cp: not All MMIFME ==> not CC

The argument tells us that many major meteor impacts are not followed by mass extinctions (i.e. not All MMIFME), which allows us to conclude with 100% certainty that we do not have a consistent causal link (i.e. not CC).

Answer choice (D) can be diagrammed as:
not CC ==> not All MMIFME

This is an incorrect reversal of the contrapositive of the correct assumption set out in answer choice (A). That is why it is wrong.

Hope this helps! Please let us know if you have any additional questions.