SAT question: How do you use "whereas?"

[Saruinity]

Hello, everyone! This is the first time I've ever been to this website and instantly I feel like that this is the place where I can learn and ask questions with no hesitations from those who are so kind to lend a helping hand. Nonetheless, I tried to answer collegeboard's "The Official SAT Question of the Day" just then but couldn't get the correct answer until a second attempt. However, I still couldn't comprehend the explanation to the correct answer.

Their question is this:
Whereas (A) their friend Sean, who often behaved as if (B)
he had only one day left to live, Rob and Darrin rarely (C) acted without first considering (D) the consequences. No error (E)

----Answer A is the correct answer and this is their explaination:
The error in this sentence occurs at (A), where inappropriate word choice creates a sentence fragment (the sentence has no main verb). To fix this problem, “Unlike” should be used in place of “Whereas.”

----The part I don't get is that what makes putting "whereas" in the original position of the sentence an inappropriate word choice? Does the sentence has to have a main verb for you to put "whereas" in a sentence?

Sorry about the wordiness of this topic, but if you know the answer to this, please reply. I greatly appreciate it! ^^[/b]

[bob@maxthetest.com]

"Whereas" means "because." It is often used in contracts and is "lawyer speak." For example, a contract may begin "Whereas John wants a new car and whereas Fred wants to sell his car, therefore...."

So if you substitute "because" for "whereas" the sentence doesn't make sense since Rob and Darrin don't act cautiously "because" Sean acts like he has only one day to live.

If I can be of further help, let me know.
_________________
aka The SAT Wizard

[Saruinity]

Thank you, Bob; I greatly appreciate it!