Explanation:
Draw a picture of the tetrahedron. Pick a point K on the line AB, then draw a triangular plane connecting the points K, C, and D.
The intersection of plane KCD and plane ABC is the line KC.
The intersection of plane KCD and plane ABD is the line KD.
The intersection of plane KCD and plane ACD is the line CD.
The intersection of plane KCD and plane BCD is the line CD.