Answer:
C. one line
Step-by-step explanation:
A plane is a two-dimensional figure which is a flat in surface.
Imagine two planes, like you can take two plain plate of the same size, when you intersect one by another by vertical, you will get a line.
Let me show you a figure here.
You can see the green line.
So the two planes insect in exactly one line.
Answer: C. One line
