这本书的最后一章以极其悲伤的基调结束。

The final chapter of the book ended on a profoundly ___sad note.

经理的公告就公司的未来发出了谨慎乐观的语调。

The manager's announcement struck a cautiously optimistic ___note about the company's future.

说点轻松的吧，让我告诉你派对上发生的那件滑稽事。

On a lighter ___note, let me tell you about the hilarious thing that happened at the party.