Remember"”Don't edit GMAC's sentences. Really, don't.
The GMAT only tests 1-2% of the things that can actually go wrong with English sentences. If random users try to edit these sentences, the result is almost always something that's incorrect"”for reasons that the GMAT doesn't test.
Making your own examples is good"”but they should be your own examples. When you make up example sentences, they should be SIMPLE sentences, meant to illustrate only one construction or concept at a time.