In the signal flow diagram given in the figure, $u_{1}$ and $u_{2}$ are possible inputs whereas $y_{1}$ and $y_{2}$ are possible outputs. When would the SISO system derived from this diagram be controllable and observable?

1. When $u_{1}$ is the only input and $y_{1}$ is the only output.
2. When $u_{2}$ is the only input and $y_{1}$ is the only output.
3. When $u_{1}$ is the only input and $y_{2}$ is the only output.
4. When $u_{2}$ is the only input and $y_{2}$ is the only output.

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