完成A接口倒换后，BSC出现2992错误（BTS AND TC UNSYNCHRONIZATION CLEAR CALLS ON A INTERFACE alarm），说明A接口可能存在哪类问题（）

A.A interface exist crossconnect circuit
B.inconsistent CCSPCM between BSC and MSC
C.A interface exist loop circuit
D.inconsistent circuit number in circuit group between BSC and MSC