有没有详细一点的答案呀？

最近遇到了一道问题，求助~~~~~~~~~~

alpha receibed a statement of account from a supplier Beta,showing a balance to be paid of $8950.Alpha's payables ledger account for Beta shows a balance due to Beta of $4140

investigation reveals the following:

1 cash paid to Beta $4080 has not been allowed for by Beta.                                                                                

2 Alpha's ledger account has not been adjusted for $40 of cash discount disallowed by Beta.

3 goods returned by Alpha $380 have not been recorded by Beta

What discrepancy remains between Alpha's and Beta's records after allowing for these items?

A   $9310

B   $390

C   $310

D   $1070

正确答案是 C   但是我不会做，谁可以告诉我解答过程呀？？小女子在这里拜谢了！