The answer is (usted) because you dont have a personal relationship with this person.
ustedes means you guys
vosotros means all of us
tu is used for someone whom you are close with like a family member or someone whom you have know for a while.
English: <span>the lava cactus consists of a number of short, cylindrical stems that grow in dense clusters, covering up to two metres.
espanol: el cactus de lava consiste en un número de tallos cilíndricos cortos que crecen en racimos densos, cubriendo hasta dos metros.
hope this helps!
espero que esto ayude!
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