Event A happens at the spacetime coordinates $(x, y, z, t)$ $=(2 \mathrm{~m}, 3 \mathrm{~m}, 0,0.1 \mathrm{~s})$ and event $\mathrm{B}$ happens at the spacetime coordinates $(x, y, z, t)=\left(0.4 \times 10^{8} \mathrm{~m}, 3 \mathrm{~m}, 0,0.2 \mathrm{~s}\right)$
(a) Is it possible that event $A$ caused event $B ?$ (b) If event B occurred at $\left(0.2 \times 10^{8} \mathrm{~m}, 3 \mathrm{~m}, 0,0.2 \mathrm{~s}\right)$ instead, would it then be possible that event A caused event $\mathrm{B}$ ? [Hint: How fast would a signal need to travel to get from event $A$ to the location of $B$ before event $B$ occurred?]