question: era:calendar:chrono
choices.label: A B C D
choices.text: stock:fund:investment dam:city wall:flood control torch:meteor:lighting mobile phone:letter:negotiation
explanation: "era" and "calendar" belong to the function of "chrono". the functions of "investment" include "stocks" and "funds", and "stocks" and "funds" are one category. the "flood control" function cannot be separated from the "dam". The "city wall" has the function of defending against foreign enemies. The "dam" and the "city wall" have different functions. the "lighting" function is inseparable from the "torch", and the "meteor" does not have the "lighting" function. one can use "mobile phone" and "letter" for communication, but not "negotiation".
relation: era calendar R2.4
relation: stock fund R2.4