How do I load parent page with <iframe loading childB.html> from the grandchildren page.

Question

Hi,

I have a parent.html, inside it, there was a iframe. When user tigger anchor link A or B, the iframe will load either childA.html or childB.html

And inside childB.html, there was a anchor link to grandchildren.html.

Currently my problem is, whenever I click the link at grandchildren.html to return to parent.html, iframe is showing childA.html

How should i do, if I want to show childB.html in iframe instead of showing childA.html

Thanks in advance

Kong