Untrue-valid argument versus Flawed arguments

[dddanny2006]

Hey guys,
I need help in understanding some of these concepts.

Source-Powerscore CR Bible
I dont seem to understand why this argument is said to be weak:

"The Jacksonville area has just over one million residents.Cincinnati has almost two million residents.The New York area has almost twenty million residents.Therefore,we should move to Jacksonville."

We clearly can figure out that the person wants to move to a low population zone,among the 3 choices that are given in the argument.How can we classify this as a weak argument?

Here's a strong argument:

"The trees that shed their foliage annually are deciduous trees.Black Oak trees shed their leaves every year.Therefore,Black Oak trees are deciduous."

If one can argue about the previous argument being weak,then we can certainly argue about this one too.Why are we talking only about Black Oak trees?or there may be other factors like- some trees only shed their leaves,while the others don't.

Please explain about identifying strong and weak arguments.

I have one more question to ask-

"All birds can fly.An ostrich is a bird.Therefore it can fly." --------1

This is an example for an argument that is valid without being true.Now what do we make of such an argument?We're supposed to accept any GMAT premise without checking them in reality.So,what if someone who didn't know the Ostrich's fly-ability,and got this question on the GMAT?

Consider the following example
"My mail was delivered yesterday,so it will also be delivered today".------------2

The book I'm referring to calls this a flawed argument,how can we be so sure?Yes,today could be a Sunday too,but we're not sure.If the GMAT tells us to accept whatever premise we get as the absolute truth,how do we disprove it?

Please tell me the difference between a Untrue-Valid argument as in 1,and the so called Flawed argument as in 2.

[RonPurewal]

Hi,
Please post this thread in the correct folder (= the general verbal folder).

Thanks.