if written as a standard AR(5)这一句话想表达什么？

问题如下：

If $(1 - 0.4L)(1 - 0.9L^4)Y_t = \epsilon_t$, what is the form of this model if written as a standard AR(5) with $Y_{t - 1},..., Y_{t - 5}$ on the right-hand side of the equation?

选项：

解释：

$(1 - 0.4L)(1 - 0.9L^4)Y_t = (1 - 0.4L - 0.9L^4 + 0.36L^5)Y_t$

$\Rightarrow$

$Y_t - 0.4Y_{t - 1} - 0.9Y_{t - 4} + 0.36Y_{t - 5} = \epsilon_t$

$Y_t = 0.4Y_{t - 1} + 0.9Y_{t - 4} - 0.36Y_{t - 5} + \epsilon_t$