Q2 An activation bar represents
the lifetime of an object
the time from when an object receives a message to when it sends a message
the time during with the object is the one performing computation
the time from when an object receives a message to when it sends a reply to that message

Q3 The order of lifelines from left to right represents:
the order in which objects send messages
the order in which objects were created
nothing except how the modeller found it convenient to draw them
the order of importance of the objects

Q4 Consider (and sketch on a piece of paper, or at least imagine doing so) a simple sequence diagram in which the messages and returns, in time order, are: (1) an actor sends m() to a; (2) a sends p() to b; (3) c sends q() to b; (4) b replies to q(); (5) b replies to p(); (6) a replies to m(). Is this legal and sensible?
No, because nothing causes q()
No, because nothing should happen in between a message and the reply to it
Yes
Yes, provided c doesn't have the same class as a