3. six transactions, t1 through t6, are requesting locks in a single-lock-mode system. here is the sequence of lock requests so far: 1. t1: lock a 2. t2: lock b 3. t3: lock c 4. t3: lock a 5. t4: lock d 6. t4: lock b 7. t5: lock e 8. t5: lock c 9. t6: lock f 10. t6: lock b draw the waits-for graph at this time. then, identify from the list below the request that will cause the requesting transaction to roll back. a) t2: lock f b) t1: lock b c) t2: lock a d) t2: lock b