A nurse is preparing regular and NPH insulin in the same syringe for a client who has diabetes mellitus. Which of the following actions should the nurse take?

Administer the mixture within 5 minutes of preparing it.

Inject air into the regular insulin vial before injecting air into the NPH vial.

Withdraw the NPH insulin before the regular insulin.

Shake both insulin vials for 2 minutes before withdrawing the doses.