Answer:
No, first is not a preposition.
If we say that outline should make use of subordination, we mean that HEADINGS SHOULD BE GENERAL WHILE SUBHEADINGS SHOULD BE MORE SPECIFIC.
In the writing process, outline helps a writer to logically and systematically arrange his materials. Each outline is expected to observe four basic principles, which are: parallelism, coordination, subordination and division. The principle of subordination requires that, In order to indicate the level of significance, an outline should use a principal and subordinate headings. Thus, materials should be ordered from general to specific.
When the soda can is plunged into cold water, the vapor condenses quickly, leaving a vacuum in most of the can. The resulting large discrepancy between the outside and inside air pressure leads to a large net inward force on the can, ending with its rapid crushing.
