A set of points $S \subseteq \mathbb{R}^2$ is convex if for any points $x, \: y \: \in S$, every point on the straight line joining $x$ and $y$ is also in $S$. For two sets of points $S, T \subset \mathbb{R}^2$, define the sum $S+T$ as the set of points obtained by adding a point in $S$ to a point $T$. That is, $S+T := \{(x_1, x_2) \in \mathbb{R}^2 : x_1=y_1+z_1, \: x_2 = y_2 + z_2, \quad (y_1, y_2) \in S, (z_1, z_2) \in T\}$. Similarly, $S-T:=\{(x_1, x_2) \in \mathbb{R}^2 : x_1 = y_1-z_1, \: x_2=y_2-z_2, \quad (y_1, y_2) \in S, (z_1, z_2) \in T \}$ is the set of points obtained by subtracting a point in $T$ from a point in $S$. Which of the following statements is TRUE for all convex sets $S, \: T$?
- $S+T$ is convex but not $S-T$
- $S-T$ is convex but not $S+T$
- exactle one of $S+T$ and $S-T$ is convex, but it depends on $S$ and $T$ which one
- neither $S+T$ nor $S-T$ is convex
- both $S+T$ and $S-T$ are convex