Past exam of the mathematics course of the University of Cambridge / 2024 / ia / Paper 4 / 4C / a / ii / Solution

False. Takewith coordinates written as $(x,t)$. Both $B$ and $C$ lie on the future lightcone of $A$, but $B$ and $C$ are simultaneous and spatially separated, so neither is in the future lightcone of the other.