Which of the Following Would Be an Example of a Research

Which of the following would be an example of a research project began as the result of a serendipitous event?

A) A suspects B of fraud, so A replicates B's work
B) A notices that complete strangers who take the same subway to work every day begin to greet one another at the station; A wonders why
C) A comes across an interesting study and does a partial replication and an extension of it
D) A has an equipment failure and notices some unusual behavior that leads to a series of studies