Valid vs. True
The LSAT is looking for valid arguments, not necessarily true ones. A valid argument follows from its premises.
An ostrich is a bird
All birds fly
Conclusion: an ostrich can fly
The above argument is valid, but not true. Try not to argue or bring in external knowledge to the LSAT, where you are just looking for valid arguments, given the information that they have given you.
Why are flying ostriches so important to skilled critical thinking?
In this chapter we regularly discuss absurd things. This isn't entirely for humorous effect. Skilled critical thinkers often employ the argument ad absurdum, which means taking a logical argument and stretching it to its breaking point to determine its validity. In this case, we use flying ostriches to show that arguments may be true but not valid.