For line A,
if we increase x by 1 unit then y increases by 4 units i.e. (1,3) and similarly another point becomes
(2,7).
For line B,
if we increase x by 1 uni then y also increases by 1 unit i.e ( 1,1) and similarly another points becomes (2,2),(3,3),(4,4), etc.
For line C,
if we increase x by 3 units then y increases by 1 units i.e.(3,1) and similarly another points becomes
(7,2) and so on.
In above lines, the value of x is exactly equal to that of y in line B.
therefore, line B has constant proportionality between x and y.
