"What are you laughing at?"

Translation: 你们在笑什么？

[Patrick_Dark]

There's no reason why this sentence should not accept "你" in lieu of "你們".

[PeaceJoyPancakes]

And it should be accepted without "在".

[Dr_Jerry]

I also was marked wrong for using 你 instead of 你们.

This is the third question in a row that I got wrong for an answer Duo should have accepted. Good lord, this is a frustrating course.

[PeaceJoyPancakes]

It gets more frustrating as the months wear on, with improvements seemingly having ground to a halt.

[Burkeburkeburke]

Can a native speaker comment on whether the 在 is necessary?

[PeaceJoyPancakes]

In the meantime, check out this video at 6:05, where the gamer says "笑什么？！你笑什么？！你别笑了！":

• https://www.bilibili.com/video/av17700529/

[Rayna314]

It is not necessary in everyday speech. But perhaps it is necessary to make it technically grammatically correct (I'm not sure)