After aging Accounts Receivable, Z Company estimates uncollectible for 2015 to be $4,000. The balance in the allowance account at January 1, 2015 was a $300 credit. During the year, Z Co. wrote off $400 of accounts receivable. The debit to bad debts expense for 2015 would be for what amount?

A) $3,700

B) $3,900

C) $4,000

D) $4,100

PLEASE EXPLAIN, THANKS IN ADVANCE.