doubts regarding A# on e-filing the I-539 FORM

[sand500]

i am applying for an extension of visitor's visa for my mother. I have a couple of questions regarding that:

1) Can i file the i-539 form online?

2) On the online application for I-539 The a# is not an optional field.
What do i enter for the A#? The field does not accept 'Not applicable' as a response

3) Is there an A# for a B2(VISITOR VISA holder)?

Thanks in advance for your advice.

 

[Triple Citizen]

B-2 status holders do not have A#s.