In a parallelogram, opposite angles are congruent.
This means that the four angles of the parallelogram are:

Moreover, the sum of the interior angles of a polygon with n sides is 180(n-2). So, the sum of the interior angles of a parallelogram is 360.
This means

And since
, we have 
Yes it is :)
If there is an outlier on the lower end of the graph the mean will go down. If there is an outlier at the higher end of the graph the mean will go up.
A.0 I don't actually know
B. triangle ABC is isosceles. The angles add up 180 degrees.