After school, he either goes to the library to read books or goes to science and technology group activities. This is a complex sentence expressing a choice relationship. According to the relevant information displayed in the query, the complex sentence indicating the selection relationship has an undecided choice, indicating that both the former and the latter situations occur and do not exist at the same time. There is a choice with a determined trade-off, which means that only one of the two situations is more suitable and a trade-off has been made.