Compensation Point for Light (of photosynthetic plants) is the intensity of light at which the rate of carbon dioxide uptake (photosynthesis) is exactly balanced by the rate of carbon dioxide production (respiration) or equivalently, the light intensity at which the rate of oxygen production is exactly balanced by the rate of oxygen consumption.
Answer: Where would the blood come from if there were a blockage just posterior to the middle cerebral artery on the left? If blood could not get to the middle cerebral artery through the posterior circulation, the blood would flow around the circle of Willis to reach that artery from an anterior vessel.