For CPAs Solely Engaged in Professional Tax Practice,abiding by the Statements

For CPAs solely engaged in professional tax practice,abiding by the Statements of Standards on Tax Services (SSTS) is:

A) Mandatory because it is the only set of professional conduct rules that applies to them
B) Desirable, but not mandatory
C) Mandatory, when acting as a tax return preparer, but not as a tax planner
D) Mandatory, even if they do not represent taxpayers in IRS audits or administrative proceedings