If you have concerns about whether the recipient's server is set up to block messages formatted as HTML, use ____________________ as the message format.

null can be used:

(a) to indicate a variable has no real value (b) in a Boolean expression with == (c) as a placeholder (d) all of the above

A condition that allows a programmer to circumvent the private modifier and change the private instance variable is called:

(a) a copy constructor (b) a privacy leak (c) a class invariant (d) an anonymous object