What is a secondary source, and which of the following is an example of a secondary source?

B. A book analyzing the causes of the American Revolution

A secondary source interprets, analyzes, or summarizes information from primary sources. It is typically created after the event by someone who did not directly experience it.

• A. A photograph of soldiers during World War II: This is a primary source because it is a direct visual record from the time of the event.
• B. A book analyzing the causes of the American Revolution: This is a secondary source because it is an interpretation and analysis of historical events, written after the fact, often using primary sources as evidence.
• C. A handwritten letter of the 18th-century by a political leader: This is a primary source because it is an original document created by someone directly involved in the events of that time.
• D. An ancient manuscript of Egypt: This is a primary source because it is an original document from the historical period it describes.

The final answer is $\boxed{\text{B}}$.