Indicate whether each of the following items is considered a primary authority or a secondary authority.
a. Reg. §1.305-1(b).
b. Rev. Rul. 67-225, 1967-2 C.B. 238.
c. S.M. Jones and S.C. Rhoades-Catanach, Principles of Taxation for Business and Investment Planning 2016 Ed.
d. PLR 201240007
e. Helvering v. Alabama Asphaltic Limestone Co., 315 U.S. 179 (1942).
f. United States Tax Reporter Code Arranged Explanations ¶614 Gross income defined.