In a jewelry theft case in a shopping mall, the police have found out that the perpetrator must be one of the four ABCD people: during the trial, their confessions are as follows:

A said: I was in the country the day the jewels were stolen, so I couldn't steal from the mall.

B said: D is a criminal.

C said: B is a criminal. I have seen him sell jewelry.

D said: B had a grudge against me, so he set me up.

It was verified that only one of the four people was telling the truth. Can you find out who is the criminal?

I want to know who is the criminal, not who is telling the truth. 1, assuming that A is telling the truth, then: B, C and D are all lies, so it can be concluded that D is not a criminal; B is not a criminal; D is a criminal and has contradictions, so A lies (so A is a criminal).

At the same time, I learned that both B and C had lied.

Only d told the truth.

Which means a is a criminal.