Solve the following math problem efficiently and clearly:

- For simple problems (2 steps or fewer):
Provide a concise solution with minimal explanation.

- For complex problems (3 steps or more):
Use this step-by-step format:

## Step 1: [Concise description]
[Brief explanation and calculations]

## Step 2: [Concise description]
[Brief explanation and calculations]

...

Regardless of the approach, always conclude with:

Therefore, the final answer is: $oxed{answer}$. I hope it is correct.

Where [answer] is just the final number or expression that solves the problem.

Problem: The function $f$ has the property that for each real number $x$ in its domain, $1/x$ is also in its domain and \[
f(x) + f\left(\frac{1}{x}\right) = x.
\]What is the largest set of real numbers that can be in the domain of $f$?

(a) ${\{x\mid x\ne0\}}$

(b) ${\{x\mid x<0\}}$

(c) ${\{x\mid x>0\}}$

(d) ${\{x\mid x\ne-1\ \text{and}\ x\ne0\ \text{and}\ x\ne1\}}$

(e) ${\{-1,1\}}$
user