Adjustment of status application - leave blank or write N/A?

[taral]

On forms I-485, I-130, and I-864, can I simply leave spaces blank or should I write None or N/A? If I do need to write None or N/A, which one is preferred?

[AmandaPanda]

I would write N/A so that the person reviewing the application knows it was not intentionally left in answered.