The conclusion we are trying to support is: "Thus glutamate leaking from damages or oxygen-starved nerve cells is a cause of long-term brain damage resulting from strokes."
Please not that it says "A cause," not "THE cause." We are trying to strengthen the cause-and-effect relationship between glutamate and brain damage, not eliminate other possibility.
Further, answer choice C does not eliminate other possibilities. It eliminates other neurotransmitters leaking from damages brain cells. But there could be neurotransmitters coming from another source, or something besides neurotransmitters could be causing the damage.
