Tāmen sùbù-xiāngshí, què zài lǚtú zhōng chéng le hǎo péngyǒu.
They were complete strangers, yet they became good friends during the trip.
Tips
usage
素 here means 'always; from the start' (a literary usage), not 'plain' or 'vegetarian.' The idiom emphasizes that two people have never known each other at all.
