Answer: line CD (fourth choice)
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Let's go through the choices one by one
1) Segment AB is a radius, which is not a secant line. We can rule this out.
2) line DE is a tangent line which only touches the circle at exactly one spot. We need something that cuts the circle in 2 spots for it to be a secant line. This can also be crossed off the list.
3) segment HG is a chord, which is fairly close to a secant line, but it must extend infinitely in both directions. In other words, it needs to be a line instead of a line segment. This is crossed off the list.
4) line CD is a secant line. It is a geometric line in that it goes on forever in both directions (it's not a segment, and not a ray either). Also, this line crosses the circle at 2 points, which contrasts it from a tangent line. This is the answer we want.