question: surface mail:express
choices.label: A B C D
choices.text: economy class:first class normal express train:high-speed ​​rail sms:wechat letter:fax
explanation: "surface mail" and "express" are two types of postal delivery services. "express", and "express" is faster than "surface mail".  "economy class" and "first class" are two seat classes, and "first class" is more expensive than "economy class" regardless of speed. "normal express train" and "high-speed rail" are two types of trains. "high-speed rail" speed is faster than the "normal express train". "sms" and "wechat" are two information exchange methods. There is no comparison between the speed of the two. "fax" can transmit "letters", and transmitting "letters" is the function of "fax".
relation: surface mail express R2.6
relation: economy class first class R2.4
relation: normal express train high-speed ​​rail R2.6
relation: sms wechat R2.6
relation: fax letter R3.3