- [Instructor] We are looking at question 15
in this passage about beavers.
I'll start at the beginning of the paragraph right up here.
In general, it's a good idea to start back
a sentence or two when looking at a text-based question
in case there is useful context to pick up along the way.
So, scientists have called the beaver
an ecosystem engineer because its dams
can alter the ecological makeup of its habitat.
By regulating water flow in streams,
dams have ripple effects that radiate
to all surrounding organisms.
Okay, so that's for question 15.
Our choices are all the same words,
but they have a variety of different punctuation marks.
So A is no change, that keeps the period.
B replaces it with a semicolon.
C replaces it with a comma, and D opts for an M dash.
So this is a question that's asking us
to choose the appropriate punctuation
to link these two ideas.
And whenever I see a punctuation question,
the first thing I do is look for independent clauses.
An independent clause, just as a quick refresher,
is a sentence that can stand on its own.
So it has a subject and a main verb, it's a full sentence.
Before I get going, if you want to pause the video
and try the question yourself, please take this opportunity.
Let's move on.
So I'm looking for independent clauses
on either side of the punctuation here, right?
So the first thing I'm gonna check
is the note change option.
Is there an independent clause here?
And is there an independent clause here?
By regulating water flow in streams,
I don't see a subject.
What's the thing that's doing the regulating?
And I also don't see a main verb.
There is regulating, but an ING verb needs a helper.
ING verbs need helpers like is or has been
in order to hold up a sentence.
So this first chunk here, that is a sentence fragment.
It can't stand on its own as a sentence.
It leaves us hanging.
By regulating water flow in streams, yes,
what, what happens next?
The sentence doesn't say, and that means
that it's an error we have to fix.
Cool, so we've identified that first chunk,
that's a sentence fragment.
What about the second chunk?
Is that its own sentence?
Let's see, dams have ripple effects
that radiate to all surrounding organisms.
Yes, so there's our subject, that's dams.
And there's our verb, is have.
That's all we need to focus on.
We can even ignore everything after that.
I'm just gonna cross that off.
So now we are looking for a choice
that will correct the error created in the original,
the error that results in our sentence fragment.
We have an introductory phrase
and an independent clause, and the question asks us
to find the right punctuation to connect them.
Certain punctuation marks can only follow
an independent clause, and we've established
that by regulating water flow in streams
isn't an independent clause.
So let's look at our choices and see what we've got.
Choice A is no change, and we know that's no good
because that's how we established
that there's a sentence fragment here.
Choice B has a semicolon, and semicolons
can only connect two independent clauses,
so that repeats the error
by again creating a sentence fragment.
See, now that I know what I'm looking for,
this goes relatively quickly.
We can cross that one off as well.
Choice C though, a comma, well that'll be just fine.
That's our answer, it's separating
that introductory phrase from the rest of the sentence.
I think that's the answer, but let's make sure.
Choice D is an M dash, and when an M dash is used
to connect two ideas like this,
it has to follow an independent clause, which it doesn't,
so we can cross that off and land on the one piece
of punctuation that doesn't result
in a sentence fragment, the comma.
Choice C is our answer because it's the only choice
that doesn't leave us hanging.
By regulating water flow in streams,
I am led to question what does,
and the answer is right there after the comma.
Dams have ripple effects, dams are the thing
that regulate water flow in streams.
Your strategy for any question in which your choices
are all punctuations marks should be,
check for independent clauses.
Paying special attention for semicolons,
colons, and solo M dashes.
Semicolons need independent clauses
before and after them, and colons
and solo M dashes need to come after an independent clause.
Now, a pair of M dashes is different
and behaves more like parentheses,
but that is not relevant right now.
If there's a sentence fragment,
choose the option that fixes it,
because the other options will either retain the error
or introduce a new one.