Punctuating blocks of equations

Question

What are the permissible ways to punctuate a long block of equations? In the image below, I propose 3 alternatives for a sample problem. I'd like to know which of these are acceptable and/or preferable.

Answer 1

I don't know if this is true for all journals or not, but physical review journals ask you to

"Treat the text and mathematical formulas as an entity. 
Punctuate mathematical expressions as sentences or parts of sentences."

Here is their example

See: http://home.fnal.gov/~bellanto/work/MINERvA/Phys%20Review%20Style%20Guide.pdf