Hi! Just got a new 15 MacBook Pro and thinking of getting an extra 85W magsafe power adapter. Apple.com shows the item code for the 85W as MC556LL/B and I see many on-line vendors selling MC556LL/A at a good price compared to the B model. What's the difference? Will a new 2011 MacBoo Pro work fine with the MC556LL/A or do I definitely need MC556LL/B. Thanks in advance.