What is the name for an object that enables a many-to-many relationship between two other objects?

You have a requirement to ensure that if the discount field on an opp is greater than 30%, the record should be automatically submitted for Approval.
Which of the following would help you meet this requirement? (Choose two.)

A salesperson at AW Computing only sees the Social Contacts link for Twitter and not Facebook on his records.
Why would this be happening?

What are the three different custom tabs you can create? (Choose three.)

The organization-wide default for a custom object is set to private. The Supervisor profile grants view access to the same object. A user with the Supervisor profile is also listed as the Manager on the user detail records for a subordinate. However, the Supervisor still cannot view records owned by the subordinate.
Which two issues are preventing the Supervisor from viewing records owned by the subordinate? (Choose two.)