Determine the dimension of each of the following subspaces of $\mathbb{F}^{4}$.
(a) $\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}\right) \in \mathbb{F}^{4} \mid x_{4}=0\right\}$.
(b) $\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}\right) \in \mathbb{F}^{4} \mid x_{4}=x_{1}+x_{2}\right\}$.
(c) $\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}\right) \in \mathbb{F}^{4} \mid x_{4}=x_{1}+x_{2}, x_{3}=x_{1}-x_{2}\right\}$.
(d) $\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}\right) \in \mathbb{F}^{4} \mid x_{4}=x_{1}+x_{2}, x_{3}=x_{1}-x_{2}, x_{3}+x_{4}=2 x_{1}\right\}$.
(e) $\left\{\left(x_{1}, x_{2}, x_{3}, x_{4}\right) \in \mathbb{F}^{4} \mid x_{1}=x_{2}=x_{3}=x_{4}\right\}$.