Cohesive Devices - Exercise

Serial comma or Oxford comma is the comma used just before the conjunction (and, or etc.) while listing items.  You can either use it or omit it.  The clarity of the sentence should be the priority in taking that decision.  For example

My cousins, Jeffrey and Sheena were present on the occasion.

Without serial comma, the sentence could be interpreted as saying that Jeffrey and Sheena are my cousins.  With the comma these are three different entities.

My cousins, Jeffrey, and Sheena were present on the occasion.

Reordering the list can also be used by the opposers  of serial comma.

Jeffrey, Sheena and my cousins were present on the occasion.

AP style does not require oxford comma but the Chicago Manual and MLA do.