A, if A was the first Master Detail relationship field because that will be the primary object in M:M relationship.
https://www.infallibletechie.com/2013/03/how-to-create-many-to-many-relationship.html
2. What is Merge Statement?
https://www.infallibletechie.com/2016/01/what-is-merge-statement-in-salesforce.html
https://www.infallibletechie.com/2014/08/merge-trigger-example-in-salesforce.html
3. Use of Process Builder in Salesforce
https://www.infallibletechie.com/2015/10/process-builder-in-salesforce.html
4. Custom Settings
https://www.infallibletechie.com/2013/04/custom-settings-in-salesforce.html
5. @RemoteAction in Visual force page
https://www.infallibletechie.com/2012/11/remoteaction-in-visual-force-page.html
6. What is mean by Winning Records and Losing Records in triggers?
https://www.infallibletechie.com/2016/02/what-is-mean-by-winning-records-and.html
7. What is parallel approval process in Salesforce?
https://www.infallibletechie.com/2016/02/what-is-parallel-approval-process-in.html
8. Custom Meta data in Salesforce
https://www.infallibletechie.com/2015/11/custom-metadata-types.html
9. How can we reset the governor limits within a test class?
https://www.infallibletechie.com/2016/02/how-can-we-reset-governor-limits-within.html