The basic purpose of the other$-$things$-$equal assumption is to:

a$.$ determine whether $x$ causes $Y$ or vice versa.

b$.$ allow one to focus upon macro variables by ignoring micro variables.

c$.$ allow one to reason about the relationship between variables $x$ and $Y$ without the intrusion of variable $Z.$

d$.$ allow one to focus upon micro variables by ignoring macro variables.