Secondary sources include the doctrine and the precedents, in other words, judicial decisions. They are not binding; the judge is free to use them according to his/her discretion. In our legal system, previous court decisions are not binding; but, most certainly, a judge in a lower court, may use especially the higher court decisions in similar cases. Therefore, the precedents are not binding in the Turkish legal system. However, the only exception to this rule is what is called the “unification of judgment.” Therefore, the correct option is C.