A cascade control strategy is shown in the figure below. The transfer function between the output $(y)$ and the secondary disturbance $\left(d_2\right)$ is defined as
$$G_{d 2}(s)=\frac{y(s)}{d_2(s)}.$$
Which one of the following is the $\text{CORRECT}$ expression for the transfer function $G_{d 2}(s) ?$
- $\frac{1}{(11 s+21)(0.1 s+1)}$
- $\frac{1}{(s+1)(0.1 s+1)}$
- $\frac{(s+1)}{(s+2)(0.1 s+1)}$
- $\frac{(s+1)}{(s+1)(0.1 s+1)}$