Rewrite the following sentence so it is in the passive voice:

Jack has been writing a book for six months.
A book has been being written by Jack for six months.
Complete the following task based on the instruction and optional input provided.### Instruction:
Rewrite the following sentence so it is in the passive voice:

Jack has been writing a book for six months.

### Output:
A book has been being written by Jack for six months.