An encryption rule is called:

The ciphertexts corresponding to plaintext A and B are 2A-B and 2A+B.

But according to this law, it is impossible to directly explain the example in this problem.

For example, plaintext 1 2 corresponds to cipher-3,4.

The result of using ciphertext rules directly should be 0,4.

Many candidates were forced to get the answer of 3 1 just by ciphertext 7.

But through its encryption rules, the answer should be that there is no option 2 or 3.