Structural formulas & optical activity

[PhysicsCanuck]

Homework Statement:

Which of the following compounds is not optically active?

Relevant Equations:

N/A

How do I determine which group is dash (into the page) and which is wedge (out of page)?
Won't that literally change which isomer it is?
How to determine optical activity then?
(I believe that in order for something to be optically active it has to be an enantiomer--a non-superimposable mirror image; therefore, I need to determine which of the four compounds don't fit that description.)

<<The textbook's answer is B because it is a meso compound.>>


Thank you all for your consideration and help!

 

[Borek]

Google Fischer projection.