Definition: This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.
Input: Passage: The Eritrean highway system is named according to the road classification. The three levels of classification are: primary (P), secondary (S), and tertiary (T). The lowest level road is tertiary and serves local interests. Typically they are improved earth roads which are occasionally paved. During the wet seasons these roads typically become impassable. The next higher level road is a secondary road and typically is a single-layered asphalt road that connects district capitals together and those to the regional capitals. Roads that are considered primary roads are those that are fully asphalted (throughout their entire length) and in general they carry traffic between all the major towns in Eritrea. Question: How is the Eritrean highway system named?
Output:
according to the road classification