Question

Two angles are supplementary. The first angle measures 40°. What is the measurement of the second angle?

Answer 1

The first answer would be B 140 as supplementary angles add up to 180.

The next answer would be the surface of a flat table.

Answer 2

Supplementary angles add up to 180°. 
If one is 40°, then the other is  (180° - 40°)  =  140° .

None of those choices describes a plane. 
Choice 'C' is the only example of a plane.