The process and instrumentation diagram for a feedback control strategy to maintain the level $(h)$ of a liquid by regulating a valve $(V)$ in a tank is shown below. $F_1$ is inlet liquid flow rate, $F_2$ is outlet liquid flow rate, $LT$ is the liquid level transmitter, $LC$ is the liquid level controller, $h_{sp}$ is the setpoint value of the liquid level, $h_m$ is the measured value of the liquid level and $P_v$ is the valve pressure.

The manipulating variable(s) is/are

1. $F_1$ only
2. $F_2$ only
3. $h_m$ and $P_v$ only
4. $h_{sp}$ and $P_v$ only

Answer:

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