Multi-choice problem: Continue writing the next sentence in this paragraph:

How to build an airsoft artillery piece
Make a base block.
Use a good, strong piece of wood about 2 feet (0.6 m) by 2 feet (0.6 m). This will be what everything else is built on.

pick from the following.
[1]. Your base block should be about 4 feet (1.2 m) wide and 4 feet high. This is the ideal block for an airsoft artillery piece.
[2]. ( for the mortar and mortar, make sure any building blocks that you use are adequate. ) Use polyurethane or fiberglass for medium-strength builds.
[3]. Get a 2 by 4 approximately 4-5 feet (1.2-1.5 m) long. Attach this to the top center of the 2 by 2 foot board with the end of the 2 by 4 matching up with the opposite side of the board so now it sticks out.
[4]. A base block is not a piece of wood, it is a solid piece of material like a wall or door. So make sure it is as strong as possible.
[3].