When to assume numbers to be same regardless of different letters/notations?

GRE Quant Problem Solving
1

Can we assume that w,x,y,z to be same here? If yes then SD could be same for both of the cases.
Making option D correct.

PHOTO-2024-08-26-02-49-12

Also, when to assume numbers to be same regardless of different letters/notations? like in x,y we assume being same and different while solving QC.

P.S. answer is B

2

It’s a set, so

is not possible as the standard deviation cannot be 0.