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 B? [Hint: How fast would a signal need to travel to get from event $A$ to the location of $B$ before event $B$ occurred?]