This question's explanation and your answers so far.
水田の水位を計測することによって，水田の水門を自動的に開閉するIoTシステムがある。図中のa，bに入れる字句の適切な組合せはどれか。 - IoTデバイス内には通信機能部があり，IoTサーバと相互に接続している。 - 「水田の水位」 ──▶ ［ a ］ ──▶ 通信機能部 ──▶ IoTサーバ（水位データを送信） - IoTサーバ ──▶ 通信機能部 ──▶ ［ b ］ ──▶ 「水田の水門」（開閉指示を送信） - IoTサーバ：受信した水位データを分析して，水門の開閉指示を行う。 凡例 ──▶ ：データや信号の送信方向
This option treats a as an “actuator”, but a is on the input side that measures the physical quantity of water level and turns it into data, so it should correctly be a “sensor”. Also, b's “IoT gateway” is a device that bridges and relays between IoT devices and the network (the server side); it is not the part that actually moves the water gate. The b that handles the driving should correctly be the “actuator”.
This option swaps a and b. Since a is on the measuring/input side, it is not an “actuator” but a “sensor”; since b is on the output side that receives the open/close instruction and moves the water gate, it is not a “sensor” but an “actuator”. Both roles are reversed.
a=sensor is correct, but b's “IoT gateway” is wrong. An IoT gateway is a device that relays communication between IoT devices and the server; it is not a device that physically opens and closes the water gate. The b that receives the open/close instruction and drives the water gate should correctly be the “actuator”.
正解 Pay attention to the direction of the arrows in the figure. The upper path runs “paddy field water level”→［a］→communication unit→IoT server: it measures a physical quantity in the real world (the water level), converts it into digital data, and sends it to the server. The part that handles this input is the “sensor”, so a=sensor. The lower path runs IoT server→communication unit→［b］→“paddy field water gate”: it receives the open/close instruction issued by the server and performs the physical action of actually opening and closing the water gate. The part that handles this output (driving) is the “actuator”, so b=actuator. Therefore the combination a=sensor／b=actuator, choice “エ”, is correct.