本 PDF 为作者个人存档版本，最终归档以 北京大学学位论文数据库 收录版本为准。

This is the author's archival copy of the bachelor's thesis. The canonical version is the one deposited with Peking University's thesis repository.

© 2026 宋卓洋 / Zhuo-Yang Song · CC BY-NC-ND 4.0