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first, recall what is a ray and its opposite:
If a is a line, its is the half of the line. That is, two rays are opposite rays if and only if they radiate from the same point and in opposite directions. Or, to put it another way, the angle between opposite rays is degrees.
Recall that, by definition, two objects are if they lie in the same plane. Opposite rays lie on same line; consequently, they lie in same plane too.