How should I answer the conflict of interest question during submission of paper if I have worked independently without any funding agencies and as a sole author?
If your case is a case in which conflicts of interest do not apply, then you simply state:
The author declares that she/he has no conflict of interest
assuming you are the sole author, in the appropriate section, usually stated Conflict of Interests. Note that some publishers, such as Elsevier, require you to sign a form for this purpose as well before even beginning the review process.
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