In the rectangular furnace, the grates are arranged in ___________.

In a rectangular furnace, the grates are typically arranged in tiers to allow for efficient airflow and heat distribution across the burning materials. The tiered arrangement helps to optimize combustion by providing multiple levels for the fuel to burn, enabling better utilization of the furnace's volume. This design facilitates the management of the burning process, ensuring that gases can escape and that the fuel is consumed evenly.

The other options, such as arches, rectangles, or circles, do not reflect the practical and effective design typically used in rectangular furnaces. Arches might be seen in different furnace designs, but they do not pertain to the arrangement of grates in a rectangular furnace. Rectangles and circles would not adequately support the structural and operational requirements for burning fuel effectively in this specific furnace type. The tier arrangement is therefore the most rational choice, aligning with the functional goals of fuel combustion and air circulation within the furnace.