Recovarability using log files

Consider the following log sequence of two transactions on a bank account, with  initial balance 12000,  that transfer 2000 to a mortgage payment and then apply  a 5% interest

1. T1 start
2. T1 B old=1200 new=10000
3. T1 M old=0 new=2000
4. T1 commit
5. T2 start
6. T2 B old=10000 new=10500
7. T2 commit

.

Suppose the database system crashes just before log record 7 is written. When  the system is restarted, which one statement is true of the recovery procedure?

(A) We must redo log record 6 to set B to 10500

(B) We must undo log record 6 to set B to 10000 and then redo log records 2   and 3

(C) We need not redo log records 2 and 3 because transaction T1 has committed

(D) We can apply redo and undo operations in arbitrary order because they are idempotent.