The example of a stratigraphic trap formed by a rise in sea level (paragraph 3) is brought up to make a certain point. It is used by the author of the passage principally to ________.
1) explain why sandstones covered by siltstones make an ideal reservoir and trap
2) contrast a typical stratigraphic trap with a typical structural trap
3) show why stratigraphic traps can be difficult to locate seismically
4) illustrate the point that stratigraphic traps were formed when sediments were laid down