Asymptomatic people also responsible for virus spread

The transcript discusses a study by the CDC on COVID-19 cases in Singapore from January 23 to March 16. Researchers identified 7 clusters with possible pre-symptomatic transmission, with 10 cases confirmed. A detailed examination of two clusters, E and B, highlights instances of transmission before symptoms appeared. In Cluster E, a man traveled to Japan and infected his housemate before showing symptoms. In Cluster B, patient B1 likely contracted the virus at an event and later transmitted it to patient B2 during a singing class.

What event did the woman in cluster B attend on February 15th?

When did patient B1 develop symptoms in relation to patient B2?