Explanation

A unique constraint is a rule that forbids duplicate values in one or more columns within a table. Unique and primary keys are the supported unique constraints.

Explanation

A comment is a note or annotation that an author or reviewer adds to a document.

Word displays the comment in a balloon in the margin of the document or in the Reviewing Pane.

Explanation

Circular division of disks to store and retrieve data is known as Tracks.

A track is a physical division of data in a disk drive.

Explanation

WhatsApp allows users to send text, voice messages and video messages, make voice and video calls, and share images, documents, user locations, and other content. 

Initial release date: November 2009

Original authors: Brian Acton, Jan Koum

Yahoo was founded by Jerry Yang and David Filo in January 1994 and was incorporated on March 2, 1995.

Founded: January 1994

Headquarters: Sunnyvale, California, United States

Gmail is a free email service provided by Google.

Date launched: April 1, 2004

Created by: Paul Buchheit

Explanation

The MS excel worksheet are made up of rows and columns.

In worksheet, intersection of row and column is a rectangle, it is called as cell.

Cells are used to store the numbers, value, text or formula.

Document is not a term of MS excel