① In order to measure the machine accuracy more exactly, you will measure it on a
straightedge ruler.
② To ensure it is not affected by the scratch on the table, you will measure it on a
straightedge ruler.
③ To ensure it is not affected by the dent on the table, you will measure it on a
straightedge ruler.
④ You will measure it directly on a table as it can also check the unevenness of the