A．How sea animals manage to exist.
B．How large sea animals can be.
C．How frightening the squid is.
D．How little is known about the sea.
A．Why it is difficult to use aerial photographs in research.
B．Why oceanic research is so limited.
C．How oceanic research has helped land research.
D．How fossil remains are obtained from deep sea.
A．To review what students know about volcanic activity.
B．To demonstrate the use of a new measurement device.
C．To explain the answer to an examination question.
D．To provide background for the next reading assignment.