ANS:A - rectangular notches of different sizes

 Rectangular notches of different sizes are not typically combined to form a stepped notch. Rectangular notches are typically used individually to measure flow rates in open channels. They consist of a rectangular notch or opening of a specific width over which the depth of water flowing can be measured. The depth of water over the notch is related to the flow rate through empirical formulas or tables. Stepped notches, on the other hand, are a combination of various shapes, usually rectangular and triangular notches, arranged in a stepped fashion to measure a wide range of flow rates accurately. Each step in the notch corresponds to a specific range of flow rates, allowing for more precise measurement across a broader range of flows. Therefore, the correct explanation is that a stepped notch is a combination of rectangular and triangular notches, not rectangular notches of different sizes. Thank you for your patience.