What was deposited on the bottom of the gas bottle

A．To discuss the results of the lab experiment.
B．To answer Bob’s question about the lab experiment.
C．To explain the method of collection by water displacement.
D．To prepare the students to do the lab experiment.

A．it is a new hospital and needs nurses to begin its program
B．several nurses have left recently for better jobs
C．it is receiving more patients now and needs more nurses to care for them
D．it is not in a good location and it is very difficult to get nurses to come here