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.
A．They occur at regular intervals.
B．They can withstand great heat.
C．They travel through the Earth's interior.
D．They can record the Earth's internal temperature.